A rock xylophone
Strike the rock bars one after the other. Listen to their sound. Check how the produced sound depends on the spot where you strike the bar. Find the correlation between the size of a bar and the pitch of the produced sounds.
How it works
When you strike the rock, it starts vibrating. Vibrations are the source of acoustic waves which propagate in the air and reach your ears. The smaller the rock, the higher the sound produced.
The lithophone, meaning a rock xylophone, is one of the earliest musical instruments. The oldest preserved lithophone found in Vietnam is over 6,000 years old. Instruments of this kind are still used in some regions of Asia and Africa.