Maths jokes :P

1:
A biologist, a statistician and a mathematician are sitting outside a cafe watching the world go by. A man and a woman enter a building across the road. Ten minutes later, they come out accompanied by a child.
Biologist: They've reproduced!
Statistician: No! It's an observational error. On average, two and a half people went each way.
Mathematician: No, no, no! It's perfectly obvious. If someone goes in now, the building will be empty.

2:
Why did the chicken cross the Möbius strip?
To get to the other... Um...

3:
Why should a mathematician always keep him wife and mistress around? (for gender equality put husband and lover)
So that when his mistress thinks he's with his wife and his wife thinks he's with his mistress, he can do Maths!!

4:
An engineer, a physicist, and a mathematician are staying in a hotel. The engineer wakes up and smells smoke. He goes into the hallway, sees a fire, fills the wastepaper basket from his room with water and pours it on the fire, putting it out.

Later, the physicist wakes up and smells smoke. He goes into the hallway and sees a (second) fire. He pulls a fire hose off the wall. Having calculated the temperature of the exothermic reaction, the velocity of the flame front, the water pressure in the hose, etc, he uses the hose to put out the fire with the minimum expenditure of energy.

Later, the mathematician wakes up and smells smoke. He goes into the hallway and sees a (third) fire. He notices the fire hose on the wall, and thinks for a moment… Then he says, "OK! A solution exists!" – and goes back to bed.