You know, witches can float, and they are burned. Wood can float and is burned. Therefore, witches must be made of wood. Ducks can also float, which means it must weigh the same as wood. This leads to the conclusion that if a woman weighs the same as a duck, that means she's made of wood, and therefore a witch. It's completely logical!
In England in the middle ages, suspected witches were thrown in the rivers, if they floated they were a witch and were subsequently burned, and if they didn't, they weren't a witch but were dead anyway.