I won't apologise for any of these. They're all classics.
Did you know that the "B" in Benoit B. Mandelbrot stands for Benoit B. Mandelbrot?
ex is having a drink in a pub and propping up the bar on his own. The barman walks over to him and asks him, "Look man, it's a busy bar, why are you standing drinking on your own? Why don't you integrate?"
ex looks at the barman morosely and answers, "Why would I? It wouldn't make any difference..."
A mathematician and a physicist are busy sinking a few pints in the pub, just a few metres away from ex. They're having a great time trading arguments and philosophical paradoxes. After a while the mathematician says , "Here's a tricky little problem. Imagine you're on a riverbank, and there's a beautiful woman lying on the opposite bank, naked. With your first step, you can get halfway across the river. With your second step, you can get half as far - that's another quarter, so you're three quarters of the way across. With your next step, you can only go half as far again. This carries on for ever. You'd never reach the naked woman, would you?"
The physicist thinks for a moment, then says slowly, "Perhaps not...but I'd be close enough for all practical purposes."
Finally, a quote from one of the greatest minds of the last hundred years...
"Physics is to mathematics as sex is to masturbation" - Richard Feynman
***Update*** - heard a new one over the weekend:
A Higgs boson walks into a church. "Why are you here," asks the priest? "Well, without me you can't have mass," replies the Higgs.