Meet The Parents (2000)
[Robert De Niro and Ben
Stiller driving in the car listening to "Puff the Magic Dragon"]
Greg Focker: Who'd have thought it wasn't about a dragon.
Jack Byrnes: Huh?
Greg Focker: Well some people think that 'to puff the magic dragon' means to... puff... smoke... a marijuana cigarette.
Jack Byrnes: Puff is just the name of the boy's magical dragon... You a pothead, Focker?
Greg Focker: No, I pass on grass always. Well not always.
Jack Byrnes: Yes or no?
Greg Focker: No, um, yes, um...
Source: Memorable Quotes from Meet the parents (2000) – Internet Movie Database.