Answer:
There is an ancient invention. Still used in some parts of the world today. That allows people to see through walls.
By Moon or by Sun, I shall be found. Yet I am undone, if there's no light around.
My teeth are sharp, my back is straight, to cut things up it is my fate.
This is in a realm of true and in a realm false, but you experience me as you turn and toss.
What has a bell but isn't a church. Is full of air but is not a balloon?
I am owned by Old McDonald.