Mass is the amount of matter in an object. Matter is made up of tiny particles. Since every object is made of particles, every object has mass. The more particles an object has, the greater its mass is.

Volume is the amount of space that matter takes up. It is easy to see that different objects take up different amounts of space. A baseball takes up more space than a marble, so a baseball has greater volume than a marble.

If two objects have the same volume, do they have the same mass? Not necessarily. Remember that objects are made of matter, and matter is made of particles. Two objects that are the same size can be made up of different numbers of particles. Why? In some materials, the particles are packed more tightly together.

Imagine you have two solid blocks, one made of wood and one of steel. The blocks are exactly the same size, so they have the same volume. Do they have the same mass? No. The particles in steel are packed together more tightly than the particles in wood. So there are more particles in the steel block than in the wooden block. As you can probably guess, the block of steel would weigh more than the block of wood.

In the same way, two objects with the same mass might not have the same volume. One kilogram of wool takes up a lot more space than one kilogram of marbles. They have the same mass. But the particles in wool are more spread out.