BUUURRRRNNNN HEEEERRR!!!

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!