I think you'll find I'm universally recognised as a mature and responsible adult

DOCTOR: Right then, your bedroom. Great. Let's see. You're twelve years old, so we'll stay away from under the bed. Cupboard! Big cupboard. I love a cupboard. Do you know, there's a thing called a face spider. It's just like a tiny baby's head with spider legs, and it's specifically evolved to scuttle up the backs of bedroom cupboards which, yeah, I probably shouldn't have mentioned. Right. So. What are we going to do? Eat crisps and talk about girls? I've never actually done that, but I bet it's easy. Girls? Yeah? 

KAZRAN: Are you really a babysitter? 

DOCTOR: I think you'll find I'm universally recognised as a mature and responsible adult. 

(He shows Kazran the psychic paper.) 

KAZRAN: It's just a lot of wavy lines. 

DOCTOR: Yeah, it's shorted out. Finally, a lie too big. Okay, no, not really a babysitter, but it's Christmas Eve. You don't want a real one. You want me. 

KAZRAN: Why? What's so special about you? 

DOCTOR: Have you ever seen Mary Poppins? 
KAZRAN: No. 
DOCTOR: Good. Because that comparison would've been rubbish. Fish in the fog. Fish in the clouds. How do people ever get bored? How did boredom even get invented? 
KAZRAN: My dad's invented a machine to control the cloud belt. Tame the sky, he says. The fish'll be able to come down, but only when we let them. We can charge whatever we like. 
DOCTOR: Yeah. I've seen your dad's machine. 
(A shark glides past behind the Doctor's back. The window is still open.) 
KAZRAN: What? You can't have. 
DOCTOR: Tame the sky. Human beings. You always manage to find the boring alternative, don't you? You want to see one? A fish. We can do that. We can see a fish. 
KAZRAN: Aren't you going to tell me it's dangerous? 
DOCTOR: Dangerous? Come on, we're boys. And you know what boys say in the face of danger. 
KAZRAN: What? 
DOCTOR: Mummy. 
(Later, the sonic screwdriver is dangling from a string passed through a lamp fitting in the ceiling and leading into -)