My favorite joke

A biologist, a physicist, and a mathematician are sitting in an outdoor cafe. They watch two people go into a building across the street. Shortly thereafter, three people come out.
"Hmm," says the biologist. "It looks like they reproduced."

"Nah," says the physicist. "There was obviously error in our initial measurement."

The mathematician looks up from his coffee. "Who cares? If another person goes in, it'll be empty."