A physicist, biologist, and mathematician are standing outside a bar. They see two people go in, but then later three people come out.
The physicist says, "Our first measurement was inaccurate."
The biologist says, "They must have reproduced."
The mathematician says, "If one more person goes in, the bar will be empty."
EDIT: Fixed the punchline, since I forgot what it actually was
Upvote silverspawn's post.