Fill the bottles with varying amounts of water.
Arrange the bottles in order from most to least full.
Blow across the top of each bottle and compare the different sounds you make.
Changing the amounts of air and water in the bottles lets you change the pitch—how high or low the sound is. When you blow across the tops of the bottles, you're making the air inside vibrate. In bottles with more air, vibrations are slower, so the pitch is lower.