more: http://www.metafilter.com/76377/Two-mathematicians-walk-into...
An infinite number of mathematicins walk into a bar. The first one orders a beer. The second orders half a beer. The third, a third of a beer. The bartender says, "what do you think I am, an idiot?" and kicks them all out.
An infinite number of mathematicians walk into a bar. The first one orders a beer. The second orders two beers. The third, three beers. The bartender says, "you guys owe me a twelfth of a beer."
(Explanation of that last one: http://en.wikipedia.org/wiki/1%2B2%2B3%2B4%2B... )