Chameleon Circuit

The chameleon circuit is a component of the TARDIS that enables its exterior to take on the shape of an object that blends into the environment it lands in. For example, when visiting a castle, it might become a stone column, a suit of armor, or similar decoration. These changes of appearance have no effect on the interior of the TARDIS. A damaged chameleon circuit may keep the TARDIS stuck in a single form- for example, the Doctor's TARDIS' damaged chameleon circuit leaves it stuck in the shape of a police telephone box, such as existed in the 1960s on Earth.