Here is a simple yet interesting project for children and a curiosity for adults.

Take a glass bottle and tap it with a spoon on the side of the bottle. Listen to the sound.

Now fill the glass bottle up halfway with water and tap the bottle with a spoon. Notice the change in sound?

Now take 4-6 bottles and put water in each glass bottle from a small amount to the large amount, and then tap each bottle in turn.

Note that each bottle sounds differently.

I made a mistake with my bottles, they still had their labels on them and it occurred to me that the labels would limit the sound, like sound-dampers, as is seen here in Image 1.