Explain xkcd: It's 'cause you're dumb.
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In the comic, two people are engaging in a battle with laser guns. One appears to gain the upper hand as he jumps on an obstacle, as the other's shot goes wide. He delivers the classic line "Any last words?" and is answered with the confusing phrase "Apollo retroreflectors". The earlier wild shot, reflected off the Moon, promptly lances down from space and hits him in the back.
A retroreflector is a device or surface that reflects light back towards its source. Several such devices were placed on the Moon by the American Apollo 11, 14, and 15 missions and continue to be used by scientists on Earth to measure the distance between the two bodies using laser ranging. The Soviet Lunokhod 1 and 2 rovers also carried such reflectors; attempts to use them for laser ranging were unsuccessful from 1971 to 2010, but were successfully renewed after the rovers' positions were photographed by the Lunar Reconnaissance Orbiter.
The likelihood of the wild shot being aimed at the Moon is fairly low in itself, and the probability of accidentally hitting a retroreflector on the Moon is lower still. Even if it did, it is highly unlikely that a pistol-sized generator could produce a beam coherent enough to inflict damage after traveling to the Moon and back, as lasers built for the purpose of hitting retroreflectors on the Moon typically get a return around one quadrillionth of the original beam.
The title text mentions that you would need to say "Apollo retroreflector" really fast, because from Earth you would only have about 2.5 seconds before the light is reflected back to its source.
Randall discussed the effect of hitting the Moon with lasers in What If: Laser Pointer and the likelihood of hitting a celestial object with a laser in What If: Into the Blue.
[Two people engage in battle using handheld laser guns.]
[Person 2's shot misses, Person 1 jumps on a desk.]
Person 1: Any last words?
Person 2: "Apollo retroreflectors"
Person 1: What?
[Person 1 is hit in the back by the reflected shot.]
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