Hey, it’s like that with all matter. It cannot be created nor destroyed. I could be made up of the same matter that was in a dinosaur or wooly mammoth or dodo bird. Or one of history’s great people. Water is, of course, the same way. If you were to keep track of where all the water I drank came from, you’d probably find that I’ve drank much of what was once pee.
Then there are some people who just drink pee. Skip the 2nd step. (http://www.straightdope.com/columns/961101.html)