How does inertia cause injury to a person in a car accident?

When a car is moving at a certain speed, objects in the car, including people, move at the same speed as well. When a car accident happens, for example the car crashes into a wall, the car stops moving suddenly because of the obstacle it hits. The people, however, don't meet the obstacle and keep on moving at the same speed they had been moving before the accident. As a result, these people usually hit the front car glass or the steering wheel and get injured.