Balloon expanding as it warms. The balloon on the left has been cooled to -198 degrees Celsius using liquid nitrogen. The molecules of air in this balloon have less energy when cooled and move slower, producing less pressure on the inside of the balloon, so it has collapsed to a smaller size. The balloon on the right has been allowed to warm up to room temperature and has regained its normal air volume. In this case the air molecules have more energy and move faster, therefore exerting more pressure on the inside of the balloon, making it expand to a larger size.