Mass is a measure of the amount of matter present in an object. Matter is anything that has mass and volume. Therefore, by definition, all matter has mass. The mass of a particular object is constant. The only way the mass of an object can change is if matter is added to or taken away from it. The more matter an object contains, the more massive the object is. Mass and volume are directly proportional to each other. If an object's mass is doubled, so is its volume. The Système International d'Unités (S) unit for mass is the kilogram (kg). The kilogram is a rather large quantity, however, so for smaller objects the gram (one-thousandth of a kilogram) or the milligram (one-thousandth of a gram) is used.

People often confuse an object's mass with its weight. The weight of an object is often expressed...