This stems from the very logical idea that if a person weighs the same as a duck, then that person is made of wood (because both ducks and wood float in water). And since wood burns (just like witches) then that person must be a witch, because witches are made of wood.
Therefore, the accused person(s) would be placed on a scale next to a duck, and if they balanced, the person(s) would be burned.
A witch? How do you know she's a witch?
She turned me into a newt!
I got better..
There is a simple way to figure out if she is a witch or not.
Witches burn, and what else burns?
Yes, and wood floats. What else floats?
... A duck?
Yes! So if this woman weighs as much as a duck, than she is a witch!
Where duck is used as a metaphor for something usually totally unrelated to ducks.
"Well, if it looks like a duck, swims like a duck, and quacks like a duck, it's probably a duck."
Golden Duck - when a batsman is out after only facing one ball
Royal Duck - when a batsman is dismissed by the first ball of the innings