Hi! Just so you know, you can click on the $ up there to support XKCDB. (This box will go away after you vote once.)

#5247 ⇧ (+839/-368) ⇩
<sparr> A mathematician, a physicist, and a biologist see two people walk into an empty building. A while later they see three people walk back out. The biologist says "they must have multiplied". The physicist says "no, our initial count was likely off". The mathematician nods sagely and says "actually, if one more person walks into the building then it will be empty again".