Doctor Who: 15 facts we bet you didn't know

Who better to reveal the secrets of Doctor Who than an actual doctor? Meet Doctor Who expert Dr. Piers D. Britton. He might sound like a Doctor Who expert we just totally made up, but he's a real person who wrote a book on Doctor Who titled TARDISbound and has taught several classes on the iconic show at the University of Redlands in California (if you're a hardcore Doctor Who fan who had to sit through dull college electives, you're probably feeling a surge of envy for Redlands students right now). With his extensive knowledge, Dr. Britton reveals 15 strange and fascinating Doctor Who facts that you probably did not know.

—Doctor Who showrunner Steven Moffat was initially opposed to Matt Smith's wish to wear his now-iconic bow tie.

—All the Silurians seen from 2010's "The Hungry Earth" to 2014 were played by the same three actors.

—Paul McGann is technically the longest-serving Doctor, though he appeared only once on television in 1996 (until 2013's "The Night of the Doctor"). Tom Baker is, of course, the longest-serving on television, having starred on more Doctor Who shows than any other actor.

—All of David Tennant's suits (including the jackets) are made out of off-the-rack pants.

—"The Impossible Astronaut" (2011) was the first episode filmed in the U.S. where the actors playing the Doctor and his companions actually participated in shooting; the earlier "Daleks in Manhattan" (2007) featured footage shot in New York, which was then digitally blended with the Welsh locations in which David Tennant and Freema Agyeman were shooting.

—The TARDIS has a six-sided control console because it was designed to have six operatives.

—"Rose" (2005) was the first episode ever named for a companion (though the title of the original pilot episode, 1963's "An Unearthly Child," does refer to the Doctor's earliest companion, his granddaughter Susan).

—The TARDIS wheezes and groans during landing because the Doctor leaves the brakes on.

—Two of the actors playing the Doctor have married actresses who had continuing or key roles on the series: Tom Baker was briefly married to Lalla Ward, who played the Time Lady Romana, in the early 1980s, and David Tennant is now married to Georgia Moffett, who played the Doctor's daughter, Jenny (and is, coincidentally, the real-life daughter of the Fifth Doctor, Peter Davison).

—Peter Capaldi and Karen Gillan not only both had Doctor Who roles before they were cast as, respectively, the Twelfth Doctor and companion Amy Pond, but actually appeared in the same episode.

—The 2007 episodes "Human Nature" and "Blink" were based on an original Doctor Who novel written in 1995 as part of the New Adventures series that picked up where the classic series left off, and, therefore, originally featured the Seventh Doctor, Sylvester McCoy.

—The ancient race of aliens, the Weeping Angels, was inspired by a carved figure in a graveyard that Steven Moffat used to see when he went on family holidays. The graveyard was marked "dangerous," which is what attracted Moffat's interest.

—The TARDIS looks like an old-fashioned police lock-up box because its cloaking device—the chameleon circuit—malfunctioned after the Doctor's first visit to 1963 London.

—The Doctor's sonic screwdriver has gone through multiple forms, and its functionality has changed a good deal: At times, it can do anything from triggering mines to repairing transmit beacons; at others, it can't even open a mortice lock (because it's too simple). Producer John Nathan-Turner had the sonic written out of the series in the early 1980s because he felt it made the Doctor's life too easy; for Russell T. Davies, on the other hand, it was important that, whatever challenges he faced, the Doctor wouldn't be limited by a locked door. In "The Day of the Doctor," we knew for sure we were going to see two sonics—Matt Smith's and David Tennant's, but, from publicity photographs, it looked very much as though the "War Doctor," played by John Hurt, would be rocking something much more like the versions used by Tom Baker and Jon Pertwee.

—Members of the Doctor's race, the Time Lords, can only regenerate twelve times—but there are loopholes. The limitation was first revealed in "The Deadly Assassin" (1976), but, ironically, the story also featured an attempt by the Doctor's arch-enemy, the Master, to renew himself after his thirteenth body had started to fail. In the seventh series, it turned out that the Time Lords had since bestowed on the Master a new cycle of regenerations. We also know that the Elixir of Life guarded by the Sisterhood on Karn—seen in "The Night of the Doctor"—can trigger regeneration. So, while the Doctor is the last of the Time Lords, his thirteenth incarnation was not his last.