Could use the "Witch logic" to find out
BEDEVERE: Tell me, what do you do with witches?
VILLAGER #2: Burn!
VILLAGER #1: Burn!
CROWD: Burn! Burn them up! Burn!…
BEDEVERE: And what do you burn apart from witches?
VILLAGER #1: More witches!
VILLAGER #3: Shh!
VILLAGER #2: Wood!
BEDEVERE: So, why do witches burn? [pause]
VILLAGER #3: B–… ’cause they’re made of… wood?
BEDEVERE: Good! Heh heh.
CROWD: Oh yeah. Oh.
BEDEVERE: So, how do we tell whether she is made of wood?
VILLAGER #1: Build a bridge out of her.
BEDEVERE: Ah, but can you not also make bridges out of stone?
VILLAGER #1: Oh, yeah.
RANDOM: Oh, yeah. True. Uhh…
BEDEVERE: Does wood sink in water?
VILLAGER #1: No. No.
VILLAGER #2: No, it floats! It floats!
VILLAGER #1: Throw her into the pond!
CROWD: The pond! Throw her into the pond!
BEDEVERE: What also floats in water?
VILLAGER #1: Bread!
VILLAGER #2: Apples!
VILLAGER #3: Uh, very small rocks!
VILLAGER #1: Cider!
VILLAGER #2: Uh, gra– gravy!
VILLAGER #1: Cherries!
VILLAGER #2: Mud!
VILLAGER #3: Churches! Churches!
VILLAGER #2: Lead! Lead!
ARTHUR: A duck!
CROWD: Oooh.
BEDEVERE: Exactly. So, logically…
VILLAGER #1: If… she… weighs… the same as a duck,… she’s made of wood.
BEDEVERE: And therefore?
VILLAGER #2: A witch!
http://www.youtube.com/watch?v=zrzMhU_4m-g