Here's another one to show that sound actually has matter, and can affect things.
Just as light waves do, sound waves travel in a straight line, and when they hit something, they're either absorbed or reflected. Regardless, upon meeting, they make something vibrate and send the sound off in every direction.
Cut out a penny-size hole out of the lid of an oatmeal box.
Put the lid back on your oatmeal cylinder, and place a candle a few inches away from the hole.
Remember that sound travels in a straight line, so it needs to be in front of the hole.
Tap the other end of your oatmeal box.
Here you go!
It's cool figuring out the world, isn't it?
This idea came from the book Barron's Science Wizardry for Kids.