What do you do with witches?

Yes that's right, you burn them.

And what do you burn, apart from witches?

No not more witches, it's wood.

With that information, we should be able to identify why witches burn. Because they're made of wood. So, in order to determine if a theoretical individual is indeed a witch, we must know first if they are made of wood.

Are there any specific properties of wood we could test? One might think to build a bridge out of said theoretical witch, but can you not also build a brigde out of stone?

So we must ask ourselves, does wood sink in water?

The answer is quite simply, no.

With that information, it's important not to throw said theoretical witch into a well, because if they sink, they're innocent. Instead we must investigate further. The last thing we learned was that wood floats on water. Is there anything else that floats on water?

The peasant mind might think "bread, apples, very small rocks" but this is not what we are looking for. Instead, the answer we need is a duck.

So, logically, if said person weighs the same as a duck, they're made of wood, and therefore a witch.