the Six Degrees of Kevin Bacon
Via guardian unlimited Stanley Milgram developed what he called the "Small World Hypothesis", which said, in effect, that everyone was connected to everyone else by six degrees or less - but it was the famous Six Degrees of Kevin Bacon online game site that firmly implanted the obscure scientific theory into the hive mind of the internet.
The game is easy. Take any actor, and you will be able to link him or her to Kevin Bacon by a chain of six or fewer actors who have appeared in films together. So, for example, take John Wayne. There's two degrees of Kevin there - John Wayne was in The Longest Day with Robert Wagner. He, in turn, was in Wild Things with Kevin Bacon.
So, technically, if Kevin wanted to have a beer with John, all he'd need to do is call up Robert, and get an introduction. Though, obviously, it might be a bit tricky now. But to the point.