[technical, for mathematicians and physicists]
Yvi just learnt that he has an Erdős number of 4. What does this mean? That he has collaborated with guy A3, which in turn collaborated with guy A2, which in turn collaborated with guy A1, which collaborated with Paul Erdős. Do you feel curious about your own number? Check this link from the AMS which allows to compute your collaboration distance to anybody, including of course Paul Erdős or Yvi.
This reminds me of the old six degrees of separation theory, which states that, defining a “A knows B” relation in an appropriate way, we get a graph such that the maximum distance between two people on Earth is 6. And I would bet for lower numbers even in most cases. OK, let’s say I have something extremely important to say to George W. (ok, I do, but I don’t think he’ll take the effort of listening anyhow, so this is just an academic exercise), so I calculated that also in four steps I can reach him.