Scientists' favourite jokes

The Observer has a great collection of scientists' favourite jokes, along with anaecdotes and background about each joke. I snorted at this limerick:

A friend who's in liquor production,
Has a still of astounding construction,
The alcohol boils,
Through old magnet coils,
He says that it's proof by induction.

From Helen Czerski, Institute of Sound and Vibration Research, Southampton.

The comments are also a goldmine of science groaners (and pedantic nit-picking). My own favourite -- and original -- science (well, maths) joke is:

There are 10 kinds of people in the world: people who don't know binary, people who do, and people who know ternary.

Please, give us your best science humour in the comments!

Scientists tell us their favourite jokes: 'An electron and a positron walked into a bar…' [The Observer]