The Doctor and Leela in “The Invisible Enemy”

What do you know about brains, anyway?

Leela

All right, all right, don't get excited!

I'll get excited if I want to! It's my brain! I'll tell you something about brains-- do you want to know something?

Leela

Not much.

I'll tell you anyway. Someone once tried to build a machine as efficient as the brain. The only problem was, it would have had to be bigger than London-- do you remember London?-- and powered by the entire European grid. And that was just a human brain. Mine's much more complex.