density: A substance's density is its amount of mass per unit of volume. Gases have low densities. Liquids have higher densities. Solids usually have the highest densities. A golf ball and a ping pong ball have about the same volume. The golf ball has a lot of matter packed inside. The inside of a ping pong ball is air. The golf ball has much more mass and therefore is more dense. So, the golf ball is heavier. Some material expands or contracts as the temperature changes. Extremely high pressures, such as occur inside a planet or the Sun, also affect density. The pressure squeezes matter into a smaller volume, resulting in higher density.
