Who says mathematicians have no sense of humour?  Here are a selection of my favourite maths and number-related jokes.  Read at your own risk.

There are three types of people in the world.  Those who can count, and those who can’t.

There are 10 types of people in the world.  Those who understand binary, and those who don’t.

Q. How can you prove that a cat has nine tails?
A. No cat has eight tails. Since one cat has one more tail than no cat, it must have nine tails

Why did the mathematical tree fall over? Because it had no real roots.

A physicist, a biologist and a mathematician are sitting in a street café watching people entering and leaving the house on the other side of the street. First they see two people entering the house.

After a while they notice three people leaving the house. The physicist says, “The measurement wasn’t accurate.”

The biologist says, “They must have reproduced.”

The mathematician says, “If one more person enters the house then it will be empty.”

Q. Why is the number six scared of seven?
A. Because seven eight nine!

Three men are in a hot-air balloon. Soon, they find themselves lost in a canyon somewhere. One of the three men says, “I’ve got an idea. We can call for help in this canyon and the echo will carry our voices far.”

So he leans over the basket and yells out, “Helllloooooo! Where are we?” (They hear the echo several times.)

15 minutes later, they hear this echoing voice: “Helllloooooo! You’re lost!!”

One of the men says, “That must have been a mathematician.”

Puzzled, one of the other men asks, “Why do you say that?”

The reply: “For three reasons. (1) he took a long time to answer, (2) he was absolutely correct, and (3) his answer was absolutely useless.”

When Noah sends his animals to go forth and multiply, a pair of snakes replies “We can’t multiply, we’re adders” — so Noah builds them a log table.

This one only works if you can pronounce French.  It was taught to me by a Brownie!

Q: Two cats were having a race.  One cat was called “One-Two-Three”, and the other was called “Un-Duex-Trois”.  Which cat won?
A: One-Two-Three.  Because Un-Deux-Trois quartre cinq!

Q: What do you get if you integrate 1 / cabin?
A: A beach hut.  (Explanation)

And finally, a warning about statistics and why you should not take them too literally:

A mother of three is pregnant with her fourth child.

One evening, the eldest daughter says to her dad: “Do you know, daddy, what I’ve found out?”

“No.”

“The new baby will be Chinese!”

“What?!”

“Yes. I’ve read in the paper that statistics shows that every fourth child born nowadays is Chinese…”

Do you have any mathematical jokes to share?

Sources and Further Examples:
http://www.transum.org/software/Fun_Maths/
http://www.mrbartonmaths.com/mathsjokes.htm