An oldie...

Footballer to ref: if I called you flipping idiot, would you send me off?

Ref: of course I would.

Footballer: if I thought you were a flipping idiot, would you still send me off?

Ref: no - I can't send you off for what you're thinking.

Footballer: well ref, in that case,

Spoiler:

I think you're a flipping idiot!