If you don't have a large-enough glass jar, you can remove one side of a cardboard box and replace it with a piece of acrylic or other plastic sheet. Just make sure to seal all the edges with some duct tape etc. Brett One more thing I do to explain the sound lens is use an oscilloscope and microphone to measure the speed of sound through the CO2 filled balloon. Ron Ebert , UCR Physics Department Expanding universe demo - connect a balloon to an air supply. As you slowly inflate it, paint dots on it in different colors.
Every dot moves away from every other dot, emulating the behavior of galaxies in our universe.
When deflated, the writing becomes unreadably small. Kids can then inflate them and see what they say. Connect one balloon to one end of the tube, blow it up to about three inches in diameter and clamp it so no air can flow out of the balloon. Blow up the second balloon to about six inches in diameter and slip it onto the other end of the tube. No air can flow between the balloons because of the hose clamp.