An infinite number of mathematicians walk into a bar. The first mathematician says to the bartender, “I’d like a beer, please.” The second mathematician says, “I would like a half a beer, please, sir.” The third mathematician, smiling, says, “I’d like half of a half of a beer, please, good sir!” The bartender, glancing down the line, says, “You’re all morons!”, pours two beers and walks away.

(If you don’t get it, check the tags. If you still don’t get it, well, try Wikipedia.)