Question

Why cannot a sound be heard on the moon? How do astronauts talk to one another on the surface of moon?

Answer 1

Sound cannot be heard on the surface of moon because there is no air on the moon to carry the sound waves. Hence, sound waves cannot be propagated in vacuum.
Astronauts talk to one another on the surface of moon through wireless sets that uses radio waves. This is because radio waves can travel through vacuum, whereas sound waves cannot travel through vacuum as they require material medium for their propagation.