Matter is anything or any material that exists. Matter occupies space and has mass.

Mass is the measure of the "stuff" of matter. Volume measures the space that matter occupies. Density measures how much "stuff" is in how much "space" or how tightly compacted matter is.

Measuring volume isn't always easy. Some objects have regular or uniform shapes. The volume of these objects can usually be measured with a formula.

Other objects have irregular shapes. Their "space" or volume is often measured by a system called water displacement. A certain amount of water is placed in a container and the height of the water is marked. The "irregular" object is put into the water, and the object causes the water line to rise. This new water level is marked. When the object is removed from the water, the water line goes back to its original place. Measuring how much water is needed to meet the raised water line tells you the volume of the irregular object. The difference between the first line and the raised line measures the volume of the object.

We measure liquid volume by using special tools. Measuring cups, easily found in most kitchens, measure liquid volume. Scientists measure liquid volume in graduated cylinders that have units that are marked in milliliters and liters.