Our skulls are not very thick, about the width of three stacked pennies. If a skull suffers a direct impact it can crack, and the brain floating inside the skull crashes into it, potentially causing bruising, swelling and tearing of the brain.
If you fall from your bike and your head hits the ground, it is easily travelling at 20 km/h or faster. At these speeds, your brain can receive permanent, irreversible injury.
A bike helmet works by absorbing the force of the impact and spreading it out over the whole helmet. The impact on your head and your brain is reduced.
Think about an egg. It is easy to break if you hit it at one spot. But if you hold it evenly and try to squeeze it, an egg can be very hard to break. In a similar way your bike helmet evens out the force your head feels on impact, reducing the severity of the injury to your head and your brain.
