What is mass?
Mass is the amount of matter in a substance. Matter is the atoms, ions, and molecules that make up a substance, or more specifically, all the protons, neutrons, and electrons that make up a substance. "In physics, mass is a property of a physical body which determines the body's resistance to being accelerated by a force and the strength of its mutual gravitational attraction with other bodies. The SI unit of mass is the kilogram (kg)." http://en.wikipedia.org/wiki/Mass