This is how "They" write flight manuals:
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?
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!