Erdos number

Paul Erdos (1913-1996) was a prolific hungarian mathematician of the highest calibre. He wrote more than 1500 papers, more than any mathematician who has ever lived !! He had collaborated with over 500 mathematicians in the process. This gave rise to a concept called Erdos number.

Paul Erdos has an Erdos number of 0. Anyone who has co-authored a paper with Erdos has an Erdos number of 1. If you have co-authored a paper with a person whose Erdos number is 1, then your Erdos number is 2, and so on. If you are never in the link, then your Erdos number is infinity.
More : http://www.loni.ucla.edu/~thompson/ERDOS/erdo.htm

Joke :

A mathematician, a biologist and a physicist are sitting in a street cafe watching people going in and coming out of the house on the other side of the street.
First they see two people going into the house. Time passes. After a while they notice three people coming out of the house.

The physicist: "The measurement wasn't accurate."
The biologist: "They have reproduced".
The mathematician: "If now exactly one person enters the house then it will be empty again."