Author's Notes:
Doctor/TARDIS. From a prompt at the "Merry Month of Drabbles" meme.

Kinetics: in physics, the study of motion and its causes; alternatively, in chemistry, the study of the rate at which a reaction proceeds.

Sometimes she runs on mercury—the quicksilver element of travellers and liars—and needs refilling. Sometimes she runs on the cosmic energies of the Eye of Harmony, or the leakage from the Cardiff Rift. Sometimes she's steered by typing in coordinates, like a computer; sometimes by playing chords on arrays of shiny buttons, like the pipe organ she once turned into as a joke; sometimes it's a dance, an argument, a song.

But always they bounce off each other, always they return; always, always, whatever forms she and her thief borrow, whatever interactions they mimic, they move as a pair.