When heated, most matter expands, or gets larger. This is why basketballs bounce much higher on hot days. Most matter shrinks, or contracts, when cooled. This is why your bicycle tires might appear flat on a cold day. The particles making up matter are constantly colliding with one another. The particles move faster as more energy is added. This makes the particles bounce off each other with more energy. Since the particles collide with more energy, they need more space. This makes matter expand.