Imagine—it's the mid-1800s, and you're riding your high-wheeled, penny-farthing bicycle down a dusty road. Sure, it may have some bumps, but if you lose your balance, you're landing on a relatively soft dirt road. But as the years go by, these roads are replaced with pavement, cobblestones, bricks or wooden slats. All these materials are much harder and still quite bumpy.
