Twenty One Tumbler Security System

The Twenty One Tumbler Security System was a security measure that the Doctor's TARDIS was originally equipped with until it was removed by the Doctor. The final step involved throwing a brick through the window of the TARDIS door and flicking a catch to open it. The Fifth Doctor reinstated the system in the presence of Tegan Jovanka with his demonstration leaving her less than impressed. (NOTVALID: Doctor Who? 88)