Answer:
Man walks over, and man swims under. In times of war, it can be burned asunder. What is it?
Add a T and I shoot through the sky. Then add an S and I multiply. What word is it?
My teeth are sharp, my back is straight, to cut things up it is my fate.
I don't have eyes, but once I did see. Once I had thoughts, but now I'm white and empty.
It cannot be seen, cannot be felt, Cannot be heard, cannot be smelt. It lies behind stars and under hills And empty holes it fills. It comes first and follows after, Ends life, kills laughter.
You use it between your head and your toes, the more it works the thinner it grows.