Probably apocryphal, but the best one I heard (and I suspect it would be from Valley):
QFI: "Bloggs, this is how much capacity you need to be a fast-jet pilot in the Royal Air Force."
(draws large 18" square on whiteboard)
"This is how much capacity you have."
(draws 1" square inside previous 18" square)
"Bloggs, you will never be a fast-jet pilot."
Cue Bloggs probably blubbing, and with good reason!