The joke going around work*:
An infinite number of mathematicians walk into a bar.
The first one says, “Give me a beer.”
The second one says, “Give me half a beer.”
The third one says, “Give me a quarter of a beer.”
The bartender pours two beers, slaps them onto the bar and says, “You guys should know your limits.”
Update: It does not get any funnier than this–the math in the joke explained with a straight face.
*I know, I know. It is old and not that funny. The thing is I love that joke.