Good Riddles

Question: There is a chain nailed to the wall. The chain is 10 feet long and the center of the chain dips down 5 feet from where each side of the chain is nailed to the wall. How far are the 2 ends of chain from each other?

Question: The more you add, the less it weighs.

Question: How do you make the number one disappear by adding to it?

Question: When do you go at red, but stop at green?

Question: I am an instrument that you can hear, but you can not touch or see me.

What am I?