No sound can be heard on the moon. Why?

Sound is a mechanical wave, which needs the vibration of molecules to propagate from one place to another. It requires a medium (water, steel, air, etc) to propagate. In simple terms, it just requires particles/atoms/molecules that are touching one another.

The Moon has no atmosphere like earth, so it is open in space, and space is mostly a vacuum. Thus, there is nothing to vibrate there and hence we can't hear any sound on the moon.