CAUTION: If you are a tree-hugging greenie-type, don't read any further.
Q: How do you make a bear cross?
A: Nail two bears together.
I liked that one. What makes this humour? The juxtaposition of expected context and answer, aided by that wonderful thing, the English Language. It's a little obscure, but not quite so much as the following classic:
Q: Why did the boy fall off his bike?
A: He was hit by a fridge.
Right, I'll leave you alone now.