Symmetric Graph
xkcd is great today, but most of you probably won't understand. I wouldn't either, except my combinatorics teacher last year told us stories about Paul Erdos. (Look at the hover text on the comic image.)
Paul Erdos was a mathematician famous for being prolific. He wrote (according to Wikipedia) over 1,500 articles with 511 individual collaborators. If you wrote a paper with him, you have an Erdos number of 1. If you wrote a paper with someone who wrote a paper with him, you have an Erdos number of 2.
This just in: Tom Lehrer and Noam Chomsky have the same Erdos number. (Here is a tool for calculating Erdos numbers; it is not exhaustive.)