Why don't astronauts hit there head on the ceiling of falling shuttle?

Astronauts don't hit their heads on the ceiling of a falling shuttle because they experience microgravity, which creates a sensation of weightlessness. In this state, both the astronauts and the shuttle are in free fall together, moving at the same rate and direction. This means that there is no relative motion between the astronauts and the shuttle interior, preventing them from contacting the ceiling.