Mickey Smith: Wait, the Tardis, you can't just leave it. Doesn't it get noticed?
Jack Harkness: Yeah, what's with the police box? Why does it look like that?
Rose Tyler: It's a cloaking device.
Doctor: It's called a chameleon circuit. The Tardis is meant to disguise itself wherever it lands. Like, if this was ancient Rome, it'd be a statue on a plinth or something. But I landed in the 1960s, it disguised itself as a police box and the circuit got stuck.
Mickey Smith: So, it copied a real thing? There actually was police boxes?
Doctor: Yeah, on street corners. You could phone for help before they had radios and mobiles. If they arrested somebody, they could shove them inside till help came. Like a little prison cell.
Jack Harkness: Why don't you just fix the circuit?
Doctor: I like it, don't you?
Rose Tyler: I love it.
Mickey Smith: But that's what I meant, there's no police boxes anymore so doesn't it get noticed?
Doctor: Ricky, let me tell you something about the human race. You put a mysterious blue box slap-bang in the middle of town, what do they do? Walk past it. Now, stop your nagging, let's go and explore.
