Gas molecules are tiny particles that make up gases, which are one of the three main states of matter, along with solids and liquids. These molecules are in constant motion, moving freely and quickly in all directions. Because of this rapid movement, gas molecules can spread out to fill any container they are in, regardless of its size or shape. The behavior of gas molecules is described by the kinetic molecular theory, which explains how temperature, pressure, and volume affect gases. For example, when gas molecules are heated, they move faster, which can increase the pressure if the gas is in a closed container.