Hit a stone with a hammer.

Answer: Seeing hammers and hammering is due to the spread of light, and hearing the sound of knocking on stones is due to the spread of sound. There is a big difference in the propagation speed between the two, so when you see the hammer beating, the sound has not yet reached the human ear. When you see the next hammer raised, the sound made by the last stone just reaches the human ear, so you can hear the sound.