Just tossing it in to see if it floats?
Yeah, if it floats, she's a witch! A witch!!... Burn her!!!
Villagers: (enter yelling) A witch! A witch! We've found a witch! Burn her! Burn her!
(After Sir Bedimere gets the crowd to admit that they dressed her up as a witch, their only basis for accusing her is that one of them claims that she turned him into a newt. But because he "got better", they need some way of determining her guilt).
Bedimere: there are ways of telling if she's a witch. What do you do with witches?
Villagers: Burn them!
Bedimere: And what do you burn, apart from witches?
Bedimere: Right! So why do witches burn?
Villagers: Because they're made of wood?
Bedimere: Right! . Now, what else do you do with wood?
Villagers: Build bridges with it!
Bedimere: But do we not also build bridges from stone; does wood float in water?
Bedimere: And what else floats in water?
King Arthur: (after more confused suggestions from the villagers) A duck!
Bedimere: Right! So, if she weighs the same as a duck, she'd float in water, and she must be made of wood, so.
Villagers: A witch! Burn her!
(They weigh the woman on a large scale with a duck in the other balancing basket, but inexplicably the scales do not tilt one way or the other. As the villagers drag the woman away, the witch looks at the camera and says with resignation "it was a fair court".)
Bedimere: (to King Arthur) Who are you who are so wise in the ways of science?
She’s a witch! (Burn her!!) - Monty Python: