Rose: What I don't get is when they killed the Prime Minister, why didn't they use him as a disguise?
The Doctor: He's too slim. They're big old beasts. They need to fit inside big humans.
Rose: But the Slitheen are about eight feet. How do they squeeze inside?
The Doctor: That's the device around their necks, compression field. Literally shrinks them down a bit. That's why there's all that gas. It's a big exchange.
Rose: Wish I had a compression field; I could fit a size smaller.
Harriet: Excuse me, people are dead. This is not a time for making jokes.
Rose: Sorry. You get used to this stuff when you're friends with him.
Harriet: Well that's a strange friendship.