Mickey: "The TARDIS, you can't just leave it. Doesn't it get noticed?"
Jack: "Yeah, what's with the police box, why does it look like that?"
Rose: "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 or something. But I landed in the 1960s, it disguised itself as a police box, and the circuit got stuck."
Mickey: "So it copied the real thing, there actually was police boxes?"
Doctor: "Yeah, on street corners. Phone for help before they had radios and mobiles. If they arrested somebody, they could shove 'em inside 'till help came. Like a little prison cell."
Jack: "Why don't you just fix the circuit?"
Doctor: "I like it, don't you?"
Rose: "I love it."
Mickey: "But that's what I meant. There's no police boxes anymore, so doesn't it get noticed?"
Doctor: "Rickey, 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!"