When a bicycle helmet breaks, it absorbs a "impulse"—a subsequent result of an original force. For example, when you fall off your bike, your head is pulled back by your arms, creating an impact (impulse) with the ground that causes you to move forward. Momentum helmets protect your skull from impacts with hard objects such as pavement or tree trunks. The idea is that if something hard hits your head, the helmet will absorb the shock instead of your brain. These protective devices were first developed for skiers and snowboarders. They came onto the market in the 1990s and are now required by law in many countries for cyclists under 18 years old. Although they may look like ski helmets, they work very differently. A momentum helmet has a shell that fits over your existing bicycle helmet and contains an air chamber inflated by a pump attached to the handlebar. When you fall down or hit something hard, the air inside the chamber is forced out through valves in the side of the helmet. This creates enough pressure to push your skull backward into the padding, protecting it from injury.