Smart jokes posted by (and re-used without direct acknowledgement) readers of Slashdot (/.):

#1

An astronomer, a physicist and a mathematician are traveling on a train through Scotland. Through the window of the train they notice a black sheep.

"Aha!" shouts the astronomer. "In Scotland, all sheep are black."

"Nonono, " says the physicist. "We only know that there are black sheep in Scotland, not that all scottish sheep are black."

The mathematician looks furiously at the other two and almost screams. "In Scotland there is at least one sheep with at least on black side!"

#2:

A biologist, a physicist and a mathematician are sitting at a cafe patio sipping coffee and watching the people go by. They see two people enter a nearby building. A few minutes later three people come out of the building.

'Ah,' the biologist says, 'they must have reproduced'.

'Nah,' says the physicist, 'three is within statistical error of two'.

'Well,' says the mathematician, 'one thing is for certain: if someone walks into the building now, it will be empty'.

Inspired by "A Puffed-Up Extrasolar Planet"

Edit 9/19/6: minor spelling errors