This is a game, whose title is a Mondegreen of Six Degrees of Separation, in which it is said that any actor or individual in the entertainment business can be linked through his or her film roles to Kevin Bacon within six steps. For example: George Clooney was in Ocean's Eleven with Matt Damon, Matt Damon was in The Departed with Jack Nicholson, and Jack Nicholson was in A Few Good Men with Kevin Bacon. Nicholson has a Bacon number of 1, Damon has a Bacon number of 2, and Clooney has a Bacon number of 3 (see below). It should be noted that this spreads across all actors; Megumi Hayashibara can be linked to Kevin in only two movesnote , and most Japanese voice actors can be linked to her within a few degrees. It's often a game for film nerds to test the abilities of each other.
There is a whole website, The Oracle of Bacon, dedicated to this that is found here.
Mathematicians have extrapolated this phenomenon and given it a name: "Bacon Number", which sounds like one of those impossibly high numbers with dangerous implications for the future of the Universe but is in fact incredibly low. Researchers have demonstrated (true story!) that almost everyone has a Bacon Number < 10, due to Kevin Bacon appearing in, mathematically speaking, almost every film on the planet. For instance, Kevin Bacon, himself, has a Bacon Number of 0. Chew on that, Mr. T! (The Oracle of Bacon claims that three persons on the IMDb have Bacon Numbers of 9, the highest listed, but does not say who they are.)
Curiously, the most connected person in acting is not Kevin Bacon. Research by people who found it important enough to do so discovered that the centre of the Hollywood universe shifts over time. As of February 2019 the center of the Hollywood universe was Christopher Lee, with Michael Caine #2 and Christopher Plummer third. Bacon himself isn't even all that high on the list, ranking near the middle of the top 1,000 centers of the Hollywood universe.
Scientists also have a similar ranking called the Erdős Number, based on co-authorship of mathematical articles with deceased mathematician Paul Erdős. Inevitably there's a combination, the Erdős-Bacon number, based on adding the two together. Thanks to the documentaries, and occasional extra work on math-related films, some mathematicians have EB numbers as low as 3. More surprising are actors who attack the problem from the other side. Danica McKellar (Winnie from The Wonder Years and bona-fide mathematician) and Natalie Portman (wrote a psychology paper at Harvard with an Erdős link) each have Erdős-Bacon numbers of 6. (This ties them with Richard Feynman.)
There is also now the Sabbath Number, based on how many collaborations it takes to get from a given musician to Black Sabbath. Then, of course, musicians who've appeared in films have Sabbath-Bacon numbers, and a surprising number of math-geeks turned musos have Sabbath-Erdős numbers. Then, there's the Sabbath-Bacon-Erdős number. The lowest known Sabbath-Bacon-Erdős number is 8, famously held by physicist, The Simpsons guest star, and Pink Floyd guest vocalist Stephen Hawking. Neuroscientist Daniel Levitin and inventor Ray Kurzweil have the same number as well.
Chess players have a Morphy number, for Paul Morphy, and Go players have a Shusaku number, centered on Honinbo Shusaku. Both of those players died in the late 1800s, but if you're interested in a Stiglitz number of 1, economist Joseph Stiglitz is still alive and publishing. Professional sports has the Sillinger number, for hockey player Mike Sillinger. Presumably, someone out there has a defined Sabbath-Bacon-Erdős-Morphy-Shusaku-Stiglitz-Sillinger number, note but neither the person nor the number has yet been found.
On a less specific level, the aforementioned serious research into this phenomenon actually has some importance for fields like sociology and epidemiology. In the modern global world, any living or recently deceased person on Earth can apparently be connected to any other person by an astoundingly short chain of connections. Even Amazonian tribes avoiding contact with the global modern world have the occasional crossing of paths with members of neighboring villages, which will have someone who visits the closest major city now and then. Thus, an outbreak of a virus can begin in a rural village in Africa and spread to downtown Tokyo and New York in only a few weeks, simply because statistically there are going to be a handfull of people in every major city with a particularly short chain of physical connections to that village.
Ironically, Kevin Bacon did not have anything at all to do with the movie Six Degrees of Separation. note
For applications in fiction, see One Degree of Separation.