Have you ever wondered about the mathematics behind the idea of "six degrees of separation?" The Erd's Number Project offers several fairly comprehensive lists of co-author relationships to elaborate (with a bit of humor) studies of the dynamics involved in "the collaboration graph," which the website says is "a 'real-life' fairly large graph for combinatorialists to study." The co-author relationship list begins with the Hungarian mathematician Paul Erd's and branches outward, so that anyone who co-authored with Erd's is assigned Erd's number 1 and anyone who co-authored with an Erd's number 1 is assigned the Erd's number 2, and so on. The website offers some suggestions for how the lists might be used, including finding your own Erd's number, testing algorithms, or just getting a sense of the different areas of mathematics represented by Erd's' co-authors. Visitors can also learn more about Erd's, read articles about collaboration in mathematics, or browse through the websites which are linked from the co-author data lists.