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