The force an object has because of the amount of mass it has. Earth's gravity pulls on you. If you try jumping in the air, earth's gravity will pull you back to the ground. When you throw a ball gravity pulls the ball to the ground. The harder you throw the ball, the faster if moves, the longer it can fall away from the Earth. (Parents: A satellite is falling around the earth, see also orbital mechanics.)
The measure of an objects inertia, and the amount of matter (Stuff) something is made of. Not to be confused with weight, which is how heavy something is in a gravitational field (the more mass the more weight). (Parents: Try asking your child which will bounce higher: a super ball or a beach ball. The super balls are smaller but bounce higher than a beach ball even though the beach ball is much larger. I hope this does not confuse them with density. Earth is more dense than Jupiter but Jupiter has more mass.)
Energy released by atomic level changes...Creates a whole lot of energy!