Action:
Little white balls rise and fall in a tall cylinder while spectators are trying to guess the reason for the fascinating motion.

You Need:
Ten grams marble chips; five grams ordinary salt; dilute hydrochloric acid; moth balls; tail cylinder or beaker; food coloring.

Why:
Carbon dioxide gas accumulates on each moth ball. In time the gas bubbles will have sufficient buoyancy to lift the moth ball to the surface. Loss of gas at the surface causes the moth balls to sink. This movement continues for hours or days.

How:
Into the tall cylinder or beaker place the marble chips, salt and 20 ml. of acid. Add a few drops of food coloring and water to within an inch of the top. Drop in several moth balls. After several minutes, they will begin to rise and hilt Stir more salt into the solution if movement does not begin.

Suggestions:
Food coloring, red, green, blue or yellow, merely adds to the beauty of the demonstration.
A cylinder located on each end of the table with movement throughout the performance of a magic show adds to the interest. 
You can use this demonstration on a small scale as a centerpiece on the table. Use vinegar and soda to generate the gas.
To obtain a solution of correct density, add salt until the moth balls begin to float. Then add a little more water.
To add interest to the demonstration you may try this variation. Use a knife to shape the moth balls into cubes. With a pencil, put dots on each cube to make them look like dice.