xkcd.WTF!?

Explanation ➲

Cueball has obtained a gun that shoots marshmallows and promptly decides to shoot at Megan. Having taken the first few hits without much reaction, she sighs and then brings out her super soaker, which was first used on her in 220: Philosophy (and later reappears in 2334: Slide Trombone), and soaks him with it.

The next day, everyone has such guns and starts shooting marshmallows at each other. We see Beret Guy and Megan who confront Cueball, saying Hey, noob! Eat Stay-Puft®! This is like saying eat lead when threatening someone with a regular gun, since Stay Puft is a fictional brand of marshmallows from the Ghostbusters movie. (Of course, it's also a reasonable thing to say, since marshmallows are edible.^{[citation needed]}) These statements and many like them appear in many first-person shooter games with chat.

Beret Guy realizes that the "streams" of marshmallows are about to cross and shouts a warning, but it is too late and they cross anyway. This results in something gigantic appearing with a Foom Roaaar! off-screen. It roars at the three friends. Megan looks up and states that this is bad as the giant shouts You're shooting what? Presumably the crossing marshmallow beams have recreated the Stay Puft Marshmallow Man from Ghostbusters. In the movie, crossing the ghost-capturing streams from the proton packs was "bad." But in the end, in which an ancient spirit took the form of this giant Marshmallow Man, the monster was destroyed as a side effect of crossing the streams. The Stay-Puft man sees what they are shooting and is justifiably upset.

The title text is a further Ghostbusters reference, as Bill Murray was one of the actors in the movie. Since the crossing of the streams of the proton packs by Bill Murray and the other Ghostbusters is related to the destruction of the Stay Puft Marshmallow Man in the movie, the crossing of the marshmallow streams in the comic does the opposite and summons an enormous Bill Murray.