Title text: Tomorrow's sessions will be entirely devoted to sewing machine rotary hooks.
Cueball is standing at a lectern on a podium, addressing a large crowd. He is describing the program of some event, listing the different topics that will be covered. These appear to be random, but the caption gives the punchline: it is a conference on things that seem like they shouldn't work but do.
By "things that seem like they shouldn't work", it means things that the average person would have some intuitive sense that the function of thing was impossible, and yet ample real-world experience shows that they do, and may become a routine function that people depend upon. TTSLTSWBD in the title and the banner is the abbreviation for "Things That Seem Like They Shouldn't Work, But Do".
Organ transplantation, where a functioning organ is cut out of one person (possibly a dead one) and put into another person where it will now operate for their benefit. Given the very complex and delicate nature of living tissue, it's rather surprising that this could work at all. In reality, it's not a simple process, and a lot of things could go wrong, but modern medicine is advanced enough that organ transplantation is widely accepted and regularly practiced, usually functioning well enough to extend life.
Airships, or dirigibles, are huge, rigid structures which are filled with bags of lighter-than-air gas, which causes the entire structure to float, and could carry both passengers and significant loads. The idea of such a huge vessel traveling, able to both move rapidly and float in place, would be hard to imagine if it didn't exist, yet zeppelins functioned and were a practical mode of transportation for a time. Unlike the other things mentioned, airships are largely obsolete (having lost favor due to safety concerns and surpassed by other technologies). Airships are a recurring theme on xkcd.
Lunch is listed as if it was another topic of the TTSLTSWBD, but it actually just means that after discussing airships, the conference will take a break to eat lunch, as many humans usually do. Because lunch is a relatively modern construction, filling a niche that grew after dinner shifted later into the day, it may defy one's intuition. In this sense, a three-meal day may seem like it shouldn't work, but most who observe all three meals on schedule would likely argue that it does.
Mechanical gyroscopes are simple devices consisting of a spinning disc mounted inside three concentric gimbals as a fixture, or more often observed at work as a single spindle in a free-standing external frame that can be held or moved around by hand. The rotational inertia of the spinning disc resists change in orientation, and tends to remain in a single orientation (if free to do so) or else exert counter-intuitive forces (where directly encouraged to change its central axis). The notion that a disc can remain so steady can be counterintuitive even to those who understand the physical principles. This weirdness has been previously referenced in 332: Gyroscopes. An optical gyroscope does not mechanically resist any motion but (relying upon an effect originally exploited in a failed attempt to disprove Special Relativity) ultimately provides similar feedback about the rotation of the unit into which it is mounted.
Butterflies fly with an unusual fluttering pattern, which works in part due to the notoriously complex principles of fluid dynamics that may look like uncontrolled fluttering but yet somehow allows the creature to land directly on specific flowerheads to feed. This is not as intuitively understandable as the flight of larger creatures such as birds.
The title text refers to rotary hooks on sewing machines, which are a complicated (and complicated looking) mechanism whose purpose is to feed one thread in a loop around a whole spool of another thread, and are apparently counterintuitive enough that the conference feels they need a whole day to cover them.
This concept is referenced in 2115: Plutonium.
The conference that the comic pictures is another example of a thing that seems like it shouldn't work but does. At first glance, Cueball seems to be listing a random, disconnected list of topics that will be covered, which runs counter to the format of most conferences. It initially seems inconceivable that enough people would be interested in all of those separate topics for the conference to make a profit (from attendance fees). However, the audience is packed, demonstrating that this is not the case. This may be because many people enjoy the mind-expanding feeling of having their intuitions shattered.
- [Cueball stands behind a lectern on a podium, gesturing with one hand held out, speaking to an audience. A banner hangs on the wall over the crowd with large letters on it. Illegible smaller text is written under these letters.]
- Cueball: Next we have a session on organ transplants and another on airships.
- Cueball: Then lunch, then we'll have one on gyroscopes and one on butterflies.
- Banner: TTSLTSWBD
- [Caption below panel:]
- The first annual conference on Things That Seem Like They Shouldn't Work But Do
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