Gibbons, frogs, and some other types of animals develop a big bulge, or sac, in their throat when they call. These balloon-like body parts help make their call louder. Why? By pushing a lot of air into the sac, the air molecules get crowded closer together, which makes the sound louder.  Try it yourself!

What You Need:

• 2 balloons

What You Do:

• Blow up one balloon and tie it closed.
• Hold the balloon up to your ear.
• Use one finger to tap on the side of the balloon at your ear. Can you hear that it makes a louder sound?
• Put your mouth up to the side of the balloon and talk in a normal voice. The balloon will make your voice sound louder. 
• Try this! Blow up one of the other balloons, but make it bigger or smaller than the first balloon.
• Try tapping your finger against the balloon again. Does having more or less air in the balloon make the sound louder or softer?