This party game helps kids understand that which cannot be seen: longitudinal waves.
This fun activity repeats Tyndall's 1870 experiment.
Everything is made of molecules. Molecules are so small you can't see them. Even air is made of molecules. Sound travels a long distance by pushing these air molecules only a short distance. This is how longitudinal waves travel.
Demonstrate longitudinal waves with a slinky: 1) lie the slinky on its side 2) push one end to make a wave propagate down the length of the toy.
You can also demonstrate this kind of energy wave using five glass marbles: 1) line four marbles up, touching, on a flat surface and 2) flick the fifth marble at the last marble to propagate a wave.
Now for the fun part. Have the kids line up about an arm distance apart, feet hip distance apart. Place something soft, such as a mat, in front of the first child in line. Have each child place hands on the shoulders of the next child. Gently push the shoulders of the last child in line. The force will move through the line until the first child falls forward onto the mat.
WARNING: Be prepared to control excessive silliness. This is a fun activity, but you might want to have a talk about pushing first. Forego this activity if you have an excessively rowdy crowd.
Find this and other music activities for young children at http://www.boopadoo.net