In-jokes are a major part of Doctor Who and can be found in every aspect of the finished product. From the way Hand of Fear included a scene in a working quarry in reverence to the way quarries provided the backdrop for many an alien planet, to the way the final images of CGI "monsters" like Cassandra and the Gelth are vaguely reminiscent of the voice actresses who brought their words to life, subtle tributes exist through the history of the programme. There are also in-jokes you only glean from hearing about production side of things. For example, everyone picks up on Torchwood being an anagram of the letters in Doctor Who, but far fewer know that Torchwood was first used to label Doctor Who filming tapes in the early days of the Modern Who series. This bit of ingenuity was thought up out of a concern that if they were labelled Doctor Who, the tapes might be stolen because of their value. And of course, Doctor Who also has a history of not-so-subtle-really-they're-holding-a-neon-sign self-reflexive jokes, as typified by the brilliant, if slightly hokey, twentieth anniversary special, The Five Doctors. So with so much to pick from, here are five unusual, or clever, or sweet, but definitely great in-jokes from Doctor Who.