A compound is made up of two or more elements combined after a chemical reaction. The salt on your kitchen table is a compound. It is made out of sodium, which is a silvery metal, and chlorine, which is a greenish gas. But salt is not silver or green, and it's not a metal or a gas. The little white crystals of salt were made when sodium and chloride chemically reacted with each other and changed into something completely new - salt!