explain xkcd - User contributions [en]

Talk:2176: How Hacking Works

2019-07-27T07:31:45Z

172.69.33.251:

any idea what the title text is referring to? some smash mouth lyrics, maybe? [[User:Gir|-- //gir.st/]] ([[User talk:Gir|talk]]) 13:48, 15 July 2019 (UTC)
:The first line of their most popular song, All Star, is "Somebody once told me the world is gonna roll me". [[Special:Contributions/162.158.123.97|162.158.123.97]] 13:55, 15 July 2019 (UTC)
::welp, i could have figured that out! to my defense, ducking "roll them like this" only brought up the comic itself. Thanks! [[User:Gir|-- //gir.st/]] ([[User talk:Gir|talk]]) 14:06, 15 July 2019 (UTC)
: I think Randall is referring to this https://xkcd.com/792/ with himself is the "someone" who warned them. [[Special:Contributions/172.68.142.239|172.68.142.239]] 14:20, 15 July 2019 (UTC)
::I doubt that the title text was a reference to this comic, but I've linked to it in the section about the second panel. Thanks! [[User:Gir|-- //gir.st/]] ([[User talk:Gir|talk]]) 14:28, 15 July 2019 (UTC)
I always (not really) thought hacking was more like "My latest scan of the internet found a computer that hasn't updated it's OS or it's antivirus in two months!" "Score! Hack in and steal their money!" [[Special:Contributions/172.68.90.76|172.68.90.76]] 14:56, 15 July 2019 (UTC)SiliconWolf
:Usually, the scan immediately installs some virus. Noone is going to manually hacks thousands of machines. -- [[User:Hkmaly|Hkmaly]] ([[User talk:Hkmaly|talk]]) 22:43, 15 July 2019 (UTC)

Note that you can sort of get the best of both words if, instead of using just common password or just password written on paper, you use password combined from those two. Online hackers wouldn't be able to hack your other accounts because all accounts have different password, while family members wouldn't be able to hack anything because they wouldn't know the common prefix to those passwords written on paper. -- [[User:Hkmaly|Hkmaly]] ([[User talk:Hkmaly|talk]]) 22:43, 15 July 2019 (UTC)
:Nah... I just need to 1) obtain a list of leaked passwords, 2) break into your pad/cubicle to obtain your dead-tree-list of partial passwords, 3) successfully identify the fully expanded one in the former that is the superset of the appropriate partial in the latter, 4) thusly derive the likely transformation you used, 5) apply that to all the other partials to easily h4ck 4ll ov UR 07|-|3|2 |_°9!|\|5 !!!111!!11oneoneone ...6) Profit? [[Special:Contributions/162.158.34.172|162.158.34.172]] 00:09, 16 July 2019 (UTC)
::Or just drug you and beat you with a wrench until you give up the passwords [[Special:Contributions/172.68.255.14|172.68.255.14]] 13:59, 20 July 2019 (UTC)

I'm disappointed that the template at the top want authored by "The Sharpest Tool in the Shed." [[User:Mootstrap|Mootstrap]] ([[User talk:Mootstrap|talk]]) 02:39, 16 July 2019 (UTC)

Anyone has idea why Randall keeps ignoring existence of password managers? [[Special:Contributions/162.158.103.147|162.158.103.147]] 07:31, 16 July 2019 (UTC)
:Because some people don’t use them? [[Special:Contributions/172.69.33.251|172.69.33.251]] 07:31, 27 July 2019 (UTC)
I have to say, I'm not aware of anyone who thinks hacking is as Randall describes. I would have thought that most people would mistakenly think hacking is about typing complex green code onto a screen until a message pops up saying "ACCESS GRANTED". [[User:Hawthorn|Hawthorn]] ([[User talk:Hawthorn|talk]]) 12:23, 16 July 2019 (UTC)
:Well, some people put a lot of emphasis of never to write a password down, to prevent insider attacks. This leads to people reusing passwords, which leads to the much bigger risk as displayed here. So this comic is in line with [[936: Password Strength]] to keep superstition out of password policies and the attached mindsets of users. (Especially the combination of using long complex passwords, as in 936, and to never write them down would lead people to reuse them a lot.). Randall here mocks, that insider threats are only a threat with insiders (family, friends, coworkers, etc. unless you work (or use your device) in a public place, while the much bigger threat for everyone is anonymous hackers going for quantity. --[[User:Lupo|Lupo]] ([[User talk:Lupo|talk]]) 13:43, 16 July 2019 (UTC)

I have removed the claim that "Most crimes are committed by people known to their victims, [...]". This might be true for crimes committed in the real world, but I doubt that's true for "cybercrimes" (Urgh. That term!). Please do re-add it if you have a credible source for it! ;-) [[Special:Contributions/172.68.50.36|172.68.50.36]] 16:15, 16 July 2019 (UTC)

There are Smash Mouth message boards? [[User:Youforgotthisthing|Youforgotthisthing]] ([[User talk:Youforgotthisthing|talk]]) 16:28, 16 July 2019 (UTC)

First panel: Where's the fourth Teenage Mutant Ninja Turtle??[[Special:Contributions/162.158.214.82|162.158.214.82]] 11:09, 17 July 2019 (UTC)
:He's picking up pizza for the crew! [[User:Ianrbibtitlht|Ianrbibtitlht]] ([[User talk:Ianrbibtitlht|talk]]) 12:25, 17 July 2019 (UTC)

Talk:2181: Inbox

2019-07-26T20:45:53Z

172.69.33.251:

Key to the joke is that the Romans had (allegedly?) no concept of zero, i.e. Roman numerals cannot express 0.[[User:Mathmannix|Mathmannix]] ([[User talk:Mathmannix|talk]]) 18:19, 26 July 2019 (UTC)
:Maybe, but I think this refers to the Inbox Zero methodology more. [[Special:Contributions/172.68.46.209|172.68.46.209]] 19:12, 26 July 2019 (UTC)
It's a pun. It doesn't work without the notion that Romans had no concept of zero.

2179: NWS Warnings

2019-07-22T22:54:19Z

172.69.33.251:

{{comic
| number = 2179
| date = July 23, 2019
| title = NWS Warnings
| image = nws_warnings.png
| titletext = Kind of rude of them to simultaneously issue an EVACUATION - IMMEDIATE alert, a SHELTER IN PLACE alert, and a 911 TELEPHONE OUTAGE alert.
}}

==Explanation==
{{incomplete|Created by a BOT. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon.}}
This comic pokes fun at the [[wiki:National Weather Service|National Weather Service]] and its warnings, which it issues to advise of potentially severe weather impending.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}

{{comic discussion}}

Talk:2178: Expiration Date High Score

2019-07-19T06:02:14Z

172.69.33.251:

If we assume this comic is contemporary, i.e. the year she found the beans is 2019, it makes Randall's girlfriend/wife 37 years old.[[Special:Contributions/141.101.84.64|141.101.84.64]] 05:25, 19 July 2019 (UTC)
:Well... it makes Megan 37 years old. There is nothing here to strongly suggest that cueball and her are Randall and his gf/wife. --[[User:Lupo|Lupo]] ([[User talk:Lupo|talk]]) 05:54, 19 July 2019 (UTC)

When I was working at a gas station, someone brought in a propane tank which had expired in 1963 (or so). If 1963 and using this scheme, my score would be 96.6.
[[Special:Contributions/172.69.33.251|172.69.33.251]] 06:02, 19 July 2019 (UTC)

2175: Flag Interpretation

2019-07-12T22:33:03Z

172.69.33.251: /* Table */

{{comic
| number = 2175
| date = July 12, 2019
| title = Flag Interpretation
| image = flag_interpretation.png
| titletext = When Salvador Dalí died, it took months to get all the flagpoles sufficiently melted.
}}

==Explanation==
{{incomplete|Created by an IMPORTANT CLONE. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon.}}
In many countries including the United States (whose flag is depicted in the comic), it is customary to lower the flag to {{w|half staff}} when important public figures die. In the US, regulations regarding flying the flag at half staff specify the length of time for the flag to be flown at half staff, and are based on the importance of the person who has died. There are no regulations where the flag would be flown at any height other than full height or half staff, and there are no regulations where multiple flags would be flown.

Randall, as usual, makes a humorous list of fictional additional traditions.

Flying the US flag {{w|Distress_signal#Inverted_flags|upside down}} is widely considered a distress signal, and would not be intended to indicate an important figure has died. Since on average [https://www.worldometers.info/world-population/ someone dies every few seconds], it would indeed be extremely unlikely that no one would die for weeks in a row. Although not having anyone die seems good on the surface, it would trigger alarm about why this was happening; what mysterious force could possibly cause cessation of all deaths? And will it continue into the future, triggering an overpopulation crisis in short order?

The second-to-last panel is a reference to the {{w|Trolley problem}}, a well-known thought experiment in ethics: An out-of-control trolley is running toward five people who are on the tracks. If you do nothing, these five will be killed. However, you can trigger a switch that will divert the trolley onto a side track, where there is one person on the tracks who would be killed. Which is the more ethical option?

The title text is a reference to ''{{w|The Persistence of Memory}}'' and other paintings and sculptures by {{w|Salvador Dalí}} which include watches and other objects that are melting.

===Table===
{| border =1 width=100% cellpadding=5 class="wikitable"
! Flag Position !! Randall's Interpretation || Explanation
|-
|Flag at half mast
|Someone important died.
|
|-
|Flag at three-quarter mast.
|Someone died but we're not sure how we feel about them.
|Randall seems to interpret a single full-mast flag as meaning "nobody important died". If so, 3/4 mast is a compromise between that and the half mast meaning; another way to interpret it might be to say "someone half-important died".
|-
|Flag at base of the mast.
|Everyone important died.
|Likewise, if the flag is lowered halfway when one important person dies, lowering it twice as far implies that multiple important people have died. No intermediate positions are shown, so we can't be sure exactly how many.
|-
|Two flags at full mast.
|Someone important was successfully cloned.
|
|-
|Two flags at half mast.
|An important person died battling their evil clone.
|
|-
|Flag at half mast and upside-down.
|Nobody has died for weeks and that seems good but statistically it's very alarming.
|
|-
|One normal-sized flag at half mast and five tiny flags at full mast.
|Someone diverted a trolley to save five people by killing one important person.
|This is a reference to the [https://en.wikipedia.org/wiki/Trolley_problem trolley problem.]
|-
|No flag on the pole.
|The person who knows where the flag is stored at night died.
|Nobody living knows where the flag is stored, and can't seem to locate it to put it on the flagpole. Presumably the flag-keeper died at night, or the flag could still be flying. Or it may have been stolen.
|}

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
:[8 panels in 2 rows, 4 panels per row - each panel shows a flagpole in a different state of flying flag(s) with a caption at the bottom of the panel below the flagpole.]
:[The US flag at half mast.]
:Caption: Someone important died
:[The same flag at three-quarter mast.]
:Caption: Someone died but we're not sure how we feel about them
:[The flag at the base of the mast.]
:Caption: Everyone important died
:[Two identical flags at full mast.]
:Caption: Someone important was successfully cloned
:[Two identical flags at half mast.]
:Caption: An important person died battling their evil clone
:[An upside-down flag at half mast.]
:Caption: Nobody has died for weeks and that seems good but statistically it's very alarming.
:[A normal-sized flag at half mast and five tiny flags at full mast.]
:Caption: Someone diverted a trolley to save five people by killing one important person
:[A flagpole with no flag.]
:Caption: The person who knows where the flag is stored at night died.

{{comic discussion}}

2034: Equations

2018-08-17T17:00:54Z

172.69.33.251: /* Explanation */ ce

{{comic
| number = 2034
| date = August 17, 2018
| title = Equations
| image = equations.png
| titletext = All electromagnetic equations: The same as all fluid dynamics equations, but with the 8 and 23 replaced with the permittivity and permeability of free space, respectively.
}}

==Explanation==
{{incomplete|Created by an EQUATION. Do NOT delete this tag too soon.}}
This comic gives a set of equations supposedly from different areas of science in mathematics, physics, and chemistry. To anyone not familiar with the field in question they look pretty similar to what you might find in research papers or on the relevant Wikipedia pages. To someone who knows even a little about the topic, they are clearly very wrong and only seem even worse the more you look at them.

;All kinematics equations
:<math>E = K_0t + \frac{1}{2}\rho vt^2</math>
{{w|Kinematics}} describes the motion of objects without considering mass or forces.

This equation here literally states: "Energy equals a constant <math>K_0</math> multiplied by time, plus half of density multiplied by speed multiplied by time squared". The first term here is hard to interpret: it could be correct if <math>K_0</math> is a constant power applied to the system, but this symbol would more normally be used to denote an initial energy, in which case so multiplying by <math>t</math> would be wrong. The second term looks similar to the traditional kinetic energy formula <math>\frac{1}{2}mv^2</math> but with a density instead of the mass. This is then wrong without some accompanying volume term (on either side of the equation). This appears to be a play on the forumula: <math>s = ut + \frac{1}{2}\ at^2</math> where distance travelled (s) in a constantly accelerating environment, is determined by initial velocity (u), time (t) and acceleration (a)

;All number theory equations
:<math>K_n = \sum_{i=0}^{\infty}\sum_{\pi=0}^{\infty}(n-\pi)(i-e^{\pi-\infty})</math>
{{w|Number theory}} is a branch of mathematics primarily to the study the properties of integers.

Taken literally the equation says: "The nth K-number is equal to: for all i in 0 to infinity, for all pi in 0 to infinity; subtract pi from n, and multiply it with i minus e to the power of pi minus infinity". A twofold misconception can be seen here. The first is the reassignment of pi as a variable instead of the constant (3.14...). This might be a jab at how in number theory letters and numbers are used interchangeably, but where some letters are all of a sudden fixed constants. The second misconception is the use of infinity in the latter part of the formula. Naively this would signify that (with the reassigned pi values) the part in the power would range from minus infinity to zero. However, infinity is not a number and cannot be used as one without using a limit construct.

;All fluid dynamic equations
:<math>\frac{\partial}{\partial t}\nabla\cdot \rho = \frac{8}{23}
\int\!\!\!\!\!\!\!\!\!\;\;\bigcirc\!\!\!\!\!\!\!\!\!\;\;\int
\rho\,ds\,dt\cdot \rho\frac{\partial}{\partial\nabla}
</math>
{{w|Fluid dynamics}} describes the movement of non-solid material. In particular for gases, the density <math>\rho</math> is often the most interesting quantity (for liquids, this is actually just constant). A unique feature of fluid-dynamic equations is the presence of {{w|Advection|advection terms}}, which take the form of often strange-looking spatial derivatives. This equation turns this up to a new level by differentiating with respect to a differential operator <math>\nabla</math>, which does not make any sense at all. Also it has a contour integral which seems reminiscent to a closed-circle process like in a piston engine, but this does not really fit in the context (differential description of a gas), and it has a pair of {{w|Magic number (programming)|unexplained numbers}} <math>8</math> and <math>23</math>, probably alluding to the {{w|Heat capacity ratio|specific heat ratio}} which is often written out as the fraction <math>\tfrac{7}{5}</math>, whereas most other physics equations [[899: Number Line|avoid including any plain numbers higher than 4]].

The title text stating that the electromagnetism equation is the same as the fluid dynamics equation, but with the arbitrary 8 and 23 replaced with the permittivity and permeability of free space is likely because electromagnetism equations often have relations to fluid dynamics, and because those two constants appear in the vast majority of electromagnetism equations.

;All quantum mechanic equations
:<math>|\psi_{x,y}\rangle = A(\psi) A(|x\rangle \otimes |y\rangle)</math>
{{w|Quantum mechanics}} is a fundamental theory in physics which describes the nature at scales of atoms and below. It typically uses the {{w|Bra–ket notation|bra–ket notation}} in its formulae.

This equation takes a state psi in the dimensions of x and y and equates it to an operator A performed on psi multiplied by the same operator performed on the tensor product of x and y. Seeing as the state psi is already the tensor product of the states x and y, this is equivalent to performing the same unknown operator twice on psi, and unless this operator is its own inverse such as a bit-flip or Hermitian operator, this equation is therefore incorrect.

;All chemistry equations
:<math>\mathrm{CH}_4 + \mathrm{OH} + \mathrm{HEAT} \rightarrow \mathrm{H}_2\mathrm{O} + \mathrm{CH}_2 + \mathrm{H}_2 \mathrm{EAT}</math>
A {{w|Chemical equation|chemical equation}} is the symbolic representation of a chemical reaction in the form of symbols and formulae, wherein the reactant entities are given on the left-hand side and the product entities on the right-hand side. The number of each element on the left side must match them on the right side, the equation is balanced. The energy produced or absorbed in this process is not included in that formula.

This here is a modification of the combustion of methane. The correct form is often taught and a good example problem but obviously there are more chemistry problems.<math>\mathrm{HEAT}</math> is normally shorthand for {{w|activation energy}}, but in Randall's version it's jokingly used as a chemical ingredient and becomes <math>\mathrm{H}_2\mathrm{EAT}</math>, taking the hydrogen atom freed by the combustion equation shown. To deliver the punchline while maintaining proper stoichiometry, <math>\mathrm{OH}</math> (which should be <math>\mathrm{OH}^-</math>, since the oxygen keeps a free electron when it combines with a single hydrogen) is shown instead of <math>\mathrm{O}_2</math>. The proper methane combustion equation would be: <math>\mathrm{CH}_4 + 2 \mathrm{O}_2 \rightarrow 2 \mathrm{H}_2\mathrm{O} + \mathrm{CO}_2</math>

;All quantum gravity equations
:<math>\mathrm{SU}(2)\mathrm{U}(1) \times \mathrm{SU}(\mathrm{U}(2))</math>
This is more similar to expressions which appear in {{w|Grand_Unified_Theory|Grand Unified Theory}} (GUT) than general quantum gravity. Unlike some of the other equations, this one has no interpretation which could make it mathematically correct. This is similar to the notations used to describe the symmetry group of a particular phenomena in terms of mathematical {{w|Lie_Group|Lie Groups}}. A real example would be the Standard Model of particle physics which has symmetry according to <math>\rm{SU(3)\times SU(2) \times U(1)}</math>. Here, <math>\rm{SU}</math> and <math>\rm{U}</math> denote the special unitary and unitary groups respectively with the numbers indicating the dimension of the group. Loosely, the three terms correspond to the symmetries of the strong force, weak force and electromagnetism although the exact correspondence is muddied by symmetry breaking and the Higgs mechanism.

Of course, an expression missing an "=" sign, is difficult to interpret as an "equation", because equations normally express an "equality" of some kind. Nobody knows whether Randal refers to a horse here (equidae)

Randall's version clearly involves some similar groups although without the <math>\times</math> symbol it is hard to work out what might be happening. A term like <math>\rm{SU(U(2))}</math> has no current interpretation in mathematics, if anyone thinks otherwise and possibly has a solution to the quantum gravity problem they should probably get in touch with someone about that.

;All gauge theory equations
:[[File:All gauge theory equations.png]]
In physics, a {{w|Gauge theory|gauge theory}} is a type of field theory which is invariant to local transformations. The term gauge refers to any specific mathematical formalism to regulate redundant degrees of freedom.

This equation looks broadly similar to the sorts of things which appear in gauge theory such as the equations which define {{w|Yang–Mills_theory#Quantization|Yang-Mills Theory}}. By the time physics has got this far in, people have normally run out of regular symbols making a lot of the equations look very daunting. The actual equations in this field rarely go far beyond the Greek alphabet though and no-one has yet to try putting hats on brackets. The appearance of many sub- and superscripts is normal (this links to the group theory origins of these equations) and for the layperson it can be impossible to determine which additions are labels on the symbols and which are indices for an {{w|Einstein_notation|Einstein Sum}}.

The left-hand side <math>S_g</math> is the symbol for some {{w|Action_(physics)|action}}, in Yang-Mills theory this is actually used for a so-called "ghost action". On the right-hand side we have a large number of terms, most of which are hard to interpret without knowing Randall's thought processes (this is why real research papers should all label their equations thoroughly). The <math>\frac{1}{2\bar{\varepsilon}}</math> looks like a constant of proportionality which often appears in gauge theories. The factor of <math>i = \sqrt{-1}</math> is not unusual as many of these equations use complex numbers. The <math>\eth</math> symbol looks similar to a <math>\partial</math> partial derivative symbol especially as the {{w|Dirac_equation#Covariant_form_and_relativistic_invariance|Dirac Equation}} uses a slashed version as a convenient shorthand.

The rest of the equation cannot be mathematically correct as the choice of indices used does not match that on the left-hand side (which has none). In particle physics subscripts (or superscripts) of greek letters (usually <math>\mu</math> or <math>\nu</math>) indicate terms which transform nicely under Lorentz transformations (special relativity). Roman indices from the beginning of the alphabet relate to various gauge transformation propetries, the triple index seen on <math>p^{abc}_v</math> would likely come from some <math>\rm{SU(3)}</math> transformation (related to the strong nuclear force). Since <math>S_g</math> has none of these (and is thus a scalar which remains constant under these operations), we would need the right-hand side to behave in the same way. Most of the indices which appear are unpaired and so will not result in a scalar making the equation very wrong. For those not familiar with this type of equation, it is a similar mistake messing up units and setting a distance equal to a mass.

;All cosmology equations
:<math>H(t) + \Omega + G \cdot \Lambda \, \dots \begin{cases} \dots > 0 & \text{(Hubble model)} \\ \dots = 0 & \text{(Flat sphere model)} \\ \dots < 0 & \text{(Bright dark matter model)} \end{cases}
</math>
This is a parody of equations defining the {{w|Hubble's_law#Derivation_of_the_Hubble_parameter|Hubble Parameter}} <math>H(t)</math> although it looks like Randall has become bored and not bothered to finish his equation. Such equations usually have several <math>\Omega</math> terms representing the contributions of different substances to the energy-density of the Universe (matter, radiation, dark energy etc.). In this context <math>G</math> could be Newton's constant and <math>\Lambda</math> is the cosmological constant (energy density of empty space) although seeing them appear multiplied and on the same footing as <math>H</math> is unusual (the dot is entirely unnecessary). Choosing to make <math>H</math> a function of time <math>t</math> and not of redshift <math>z</math> is also unusual.

The second section looks like the inequalities used to show how what shape the Universe, based on the value of the curvature parameter <math>\Omega_k</math>. A value of 0 indicates a flat Universe (this more or less what we observe) whilst a positive /negative value indicates an open /closed curved Universe. Randall's choice of labels further makes fun of the field as both a flat sphere and bright dark matter are oxymoronic terms which would involve some rather strange model universes.

;All truly deep physics equations
:[[File:All truly deep physics equations.png]]
<math>\hat H</math> is the Hamiltonian operator, which when applied to a system returns the total energy. In this context, U would usually be the potential energy. However, there is also a subscript 0 and a diacritic marking indicating some other variable. Much of physics is based on Lagrangian and Hamiltonian mechanics. The Lagrangian is defined as <math>\hat L = \hat K - \hat U </math> with K being the kinetic energy and U the potential. Hamiltonian mechanics uses the equation <math>\hat H = \hat K + \hat U </math>. The Hamiltonian must be conserved so taking the time derivative and setting it equal to zero is a powerful tool. The principle of least action says allows most modern physics to be derived by setting the time derivative of the Lagrangian to zero.

==Transcript==
:[Nine equations are listed, three in the top row and two in each of the next three rows. For the bottom equation to the left, there are three equations in one, the part that is different in each equation is given on three lines and a bracket encompassing all three points to three dots indicating the equation continues. Below each equation there are labels:]

:E = K0t + 1/2 pvt2
:All kinematics equations

:Kn = ∑i=0∞∑π=0∞(n-π)(i-eπ-∞)
:All number theory equations

:∂/∂t ∇ ⋅ p = 8/23 (∯ ρ ds dt ⋅ ρ ∂/∂∇)
:All fluid dynamic equations

:|ψx,y〉 = A(ψ) A(|x〉⊗ |y〉)
:All quantum mechanics equations

:CH4 + OH + HEAT → H2O + CH2 + H2EAT
:All chemistry equations

:SU(2)U(1) × SU(U(2))
:All quantum gravity equations

:Sg = (-1)/(2ε̄) i ð (̂ ξ0 +̊ pε ρvabc η0 )̂ f̵a0 λ(ℨ̆) ψ(0a)
:All gauge theory equations

:H(t) + Ω + G⋅Λ ... > 0 (Hubble model) ... = 0 (Flat sphere model) ... < 0 (Bright dark matter model)
:All cosmology equations

:Ĥ - u̧0 = 0
:All truly deep physics equations

{{comic discussion}}

[[Category:Science]]
[[Category:Physics]]
[[Category:Math]]
[[Category:Chemistry]]
[[Category:Astronomy]]