Hermione: It's not going to work.
Fred: Oh yeah?
George: Why's that, Granger?
Hermione: You see this?
[gestures to a glowing circle on the floor]
Hermione: This is an age line. Dumbledore drew it himself.
Fred: So?
Hermione: So a genius like Dumbledore couldn't possibly be fooled by a dodge as pathetically dim witted as an ageing potion.
Fred: Ah, but that's why it's so brilliant!
George: Because it's so pathetically dim witted.

*I love this part!!*