"She's a witch!" from Monty Python and the Holy Grail

Timings from excerpt at

 http://www.youtube.com/watch?v=zrzMhU_4m-g

1:33 "There are ways of telling whether she's a witch" 

1:44 "What do you do with witches?"  "Burn them!"

Witch(x) => Burn(x)

2:00 "Why do witches burn?"  "'Cause they're made of wood!"

Wood(x) => Witch(x)

2:08 "How do we tell if she's made of wood?"
 
2:23 "Does wood sink in water?"  "It floats!"

Floats(x) => Wood(x)

2:29 "What also floats in water?" ... "A duck!"  2:38

Floats(duck)

2:55 "Exactly! So, logically, .... " "If she weighs the same as a duck, she's
made of wood"

 weight(x) = weight(duck)  => Wood(x)

[ from  weight(x) = weight(duck) => Floats(x) and Floats(x) = Wood(x) ]
   
3:00 "And therefore..."  "A witch!"

3:28 "Remove the supports!"

 weight(x) = weight(duck)

3:35  "A witch! Burn her!"

       ...

      "Who are you who are so wise in the ways of science?"