The Robots of Death

“To the rational mind, nothing is inexplicable; only unexplained.”

LEELA: Doctor, why didn't the helium make your voice go squeaky?

DOCTOR: Because I'm a Time Lord. I've been around, you know. Two hearts, respiratory bypass system. I haven't lived seven hundred and fifty years without learning something.

“You know, you're a classic example of the inverse ratio between the size of the mouth and the size of the brain.”