A duck!
An aquatic bird that, in medieval times, was used to find out if a person was a witch or not.

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.
So, if she weighs the same as a duck, then she's made of wood. And therefore... A WITCH!