Outer plasmic shell

The outer plasmic shell of a TARDIS was a component which could be altered in order to change the outward appearance of the TARDIS by means of its dimensionally transcendental properties. The real world interface on the outer plasmic shell enabled users to enter or leave the ship. With models such as the Doctor's TARDIS, a Type 40 TARDIS, the outer plasmic shell responded to a chameleon circuit, which configured the shape in order to best fit in with the TARDIS' surroundings. (DW: Logopolis)

Later, more sentient TARDISes might decide what to change into all by themselves. We do not know if the outer plasmic shell of a TARDIS could also change into other forms of matter, i.e. liquid or gaseous.