From a collection of scientists' favorite jokes... at The Guardian, via Metafilter, where someone says:
Aw, a lot of these are pretty weak. Reddit had a great post earlier this year, "What's the most intellectual joke you know?," that had a lot of good ones. I've stolen two of them.
The first requires some thought:
Three logicians walk into a bar. The bartender asks "Do you all want to start with a beer?"The second doesn't:
The first logician says "I don't know."
The second logician says "I don't know."
The third logician says "Yes!"
Pavlov is sitting at a pub enjoying a pint. The phone rings and he jumps up shouting "Oh shit, I forgot to feed the dog!"