The temperature of a gas is how hot or cold it is. In physics it is usually measured in kelvins although degrees Celsius are used more elsewhere. In a gas, the average velocity (how fast they move) of the molecules is related to the temperature. The faster the gas molecules are moving, the more they collide, or smash into each other. These collisions release energy, which in a gas comes in the form of heat. Conversely if the temperature around the gas becomes hotter then the gas particles will convert the thermal energy to kinetic energy, making them move faster and making the gas hotter.