According to the lastest estimate, the Internet consists of trillions of individual pages in hundreds of different languages, but a paper by Albert-László Barabási, appearing in the Philosophical Transactions of the Royal Society argues that it takes no more than 19 clicks to navigate from one page to what it estimates to be the 13,999,999,999,999 other publicly accessible web pages out there.
There are a few caveats to the paper: the author didn't actually study the real Internet — instead, he wrote a program that replicated the scale and behavior of the Internet, and studied that — and counted search engines, which require specifying what you're looking for, as a single "click." (In fact, there's an ongoing debate among computer scientists about about constitutes a click.) But the paper shows that the 19-clicks rule is inherent to the Internet's design, not an index of its current size or content. According to Smithsonian Magazine, Barabási "analyzed the network looking at a variety of levels—examining anywhere from a tiny slice to the full 1 trillion documents—and found that regardless of scale, the same 19-click-or-less rule applied."
There isn't yet a decent explanation for the 19-clicks rule — Barabási thinks it has something to do with the way pages on the Internet are grouped — but then again, there isn't yet a decent explanation for the Six Degrees of Kevin Bacon rule, either. Of course, that popular movie trivia game (1) was itself inspired by the 1993 movie Six Degrees of Separation (2), which starred Will Smith, and does not feature Bacon. The movie was based on a 1990 play of the same name written by John Guare (3), and while it's not clear where he got the idea (some say credit the Italian inventor Guglielmo Marconi), the earliest cited proponent of the theory is Hungarian novelist and poet Frigyes Karinthy (4) who is discussed in the 2002 book Linked by Albert-László Barabási (5) who today wrote about how many clicks separate web pages on the Internet (6).
See? Anyone can play this game.