You have two american coins. You add them up and you have 15 cents. One of them is not a nickel.
The other one is a nickel :]
I have no idea what a nickel is, but i've heard similar!
Ok: What goes up a chimney down, but can't go down a chimney up?
You use it from your head to your toes;
The harder it works, the thinner it grows.
What is it?