I have a firm grip on reality, now I can strangle it!

Sir Bedevere : There are ways of telling whether she is a witch.
Peasant 1 : Are there? Oh well, tell us.
Sir Bedevere : Tell me. What do you do with witches?
Peasant 1 : Burn them.
Sir Bedevere : And what do you burn, apart from witches?
Peasant 1 : More witches.
Peasant 2 : Wood.
Sir Bedevere : Good. Now, why do witches burn?
Peasant 3 : ...because they're made of... wood?
Sir Bedevere : Good. So how do you tell whether she is made of wood?
Peasant 1 : Build a bridge out of her.
Sir Bedevere : But can you not also build bridges out of stone?
Peasant 1 : Oh yeah.
Sir Bedevere : Does wood sink in water?
Peasant 1 : No, no, it floats!... It floats! Throw her into the pond!
Sir Bedevere : No, no. What else floats in water?
Peasant 1 : Bread.
Peasant 2 : Apples.
Peasant 3 : Very small rocks.
Peasant 1 : Cider.
Peasant 2 : Gravy.
Peasant 3 : Cherries.
Peasant 1 : Mud.
Peasant 2 : Churches.
Peasant 3 : Lead! Lead!
King Arthur : A Duck.
Sir Bedevere : ...Exactly. So, logically...
Peasant 1 : If she weighed the same as a duck... she's made of wood.
Sir Bedevere : And therefore...
Peasant 2 : ...A witch!
Memorable Quotes from Monty Python and the Holy Grail (1975)