[Leela is manipulating a yo-yo while The Doctor is at the control console of the TARDIS]
Doctor?
Hmm?
Can I stop now?
If you want to.
It will not affect this?
Affect this? No, it's a yo-yo. It's a game; I thought you were enjoying it.
[incredulously] Enjoying it?! [drops the yo-yo] You said I had to keep it going up and down, I thought it was part of the magic!
[looks at Leela with wide-eyed surprise] Magic, Leela? Magic?
I know; I know, there's no such thing as magic.
That's right. To the rational mind, nothing is inexplicable; only unexplained.
So; explain to me how this, "TARDIS" is larger on the inside than the out?
Hmmm? Alright, I'll show you. It's because insides and outsides are not in the same dimension. [The Doctor holds up two black cubes of differing sizes] Which box is larger?
[pointing to the larger of the two] That one.
Alright... [The Doctor places the larger cube on the command console, and walks over next to Leela and holds up the smaller one] Now, which is larger?
[incredulously, pointing at the larger cube sitting on the console] That one!
But it looks smaller.
Well, that's because it's further away!
Exactly! If you could keep that exactly that distance away, and have it here, the large one would fit inside the small one.
That's silly.
That's trans-dimensional engineering; a key Time Lord discovery!