Continuity is hard enough to keep between movies and their sequels - ask Terence Howard about Iron Man sometime, preferably from a long distance. When it comes to Doctor Who, a show that has lasted more than fifty years, and which originally aired before anyone had any conception of VCRs, the Internet, or even re-runs, it's damned near impossible. So the showrunners have essentially given up trying too hard - the invention of the phrase "wibbly wobbly, timey-wimey" to explain away any and all inconsistencies was a stroke of genius, even if the show does too frequently feel like it's trying to retroactively rewrite its own history at times (seriously, Clara was prophesied before William Hartnell's Doctor ever left Gallifrey?).But sci-fi fans - being sci-fi fans - want answers. And in the absence of specifics, they'll make them up themselves. Sometimes, people involved with the show itself make them up. Other times, devotees imagine where the universe of the show might cross into others. Here, with a debt of thanks to my Facebook friends, are 12 of the best. Be these theories frivolous, weighty, semi-canon, or imagined subtext... all of them make sense to one extent or another.