TARDIS key

The TARDIS key is a fictional item that is featured in the British science fiction television series Doctor Who. It is a rather nondescript item, but it is vitally important to the time traveler known as the Doctor, for it is what he uses to lock and unlock his time machine, the TARDIS.

History
When the Third Doctor first emerged, he knew that he was in a weakened state and might be discovered, so he hid his TARDIS key inside of his shoes. He was taken to Ashbridge Cottage Hospital and his shoes were taken away from him. He made repeated requests for them to be returned to him, after which he retrieved the key.

The Ninth Doctor took on a young companion named Rose Tyler. After journeying with her for only a few weeks, he decided that he trusted her enough to give her an extra key to the TARDIS.