The bicycle is a simple machine, but it has changed how people travel. The first bicycles of the early nineteenth century had no pedals; riders pushed along the ground with their feet. Pedals were added later, and by the end of the century the 'safety bicycle', with two wheels of the same size and a chain, had become popular. Bicycles gave ordinary people, including many women, a cheap way to travel farther than they could on foot. Today, cycling is encouraged for several reasons. It does not produce smoke, it keeps the rider fit, and it costs little compared with owning a car. Many cities have built special lanes so that cyclists can ride safely away from heavy traffic. Still, riders should always wear a helmet, because even a small fall can cause a serious injury to the head.

Question: Why does the writer advise riders to wear a helmet?