When large pieces of Ivory soap are heated in the microwave, the soap expands and changes shape until it looks like a puffy cloud sculpture. Be sure to heat the soap on a paper towel, paper plate, or shallow plastic container, not directly on the microwave floor. Allow the soap to cool for about a minute, and it firms up. The expansion is caused by molecules of water trapped in tiny air pockets in the soap. When heated, the water turns to steam and pushes against the walls of the soap, causing it to expand into a new shape. The same principle works on a popcorn kernel, which also has moisture trapped in air pockets. Since the outside of the kernel is hard (unlike the soap), the pressure builds inside until the shell can’t take it anymore, and the kernel pops. Ivory, which less density than most soaps (it floats when other soaps do not), works best for this experiment, but you can try it with different brands of bar soap to see what happens.
How you set up your science fair booth depends on the requirements of your particular fair. You may not need more than the soap and a microwave, or you may need visual aids. Posters outlining the scientific principles involved in your experiment give observers something to look at before the experiment and may help the presenter remember key points to make. Include pictures and graphics on your poster to make it interesting. The table can hold smaller displays that help demonstrate elements of your experiment. A series of clear cylinders filled with water and a different bar of soap will show that most soaps sink. You can have a bar of each soap cut in half in front of each cylinder, so observers can check for air pockets, or you can cut the soap as part of your demonstration later. You might also want to include popcorn kernels in your display. Glue a single kernel on one half of a paper plate and a popped kernel on the other, so you have something to point to when comparing the soap to the popcorn. You can also glue the kernels on to a poster.
Though you can conduct the experiment in a few minutes with one microwave, you could have two microwaves instead. Put Ivory soap in one oven and another brand in the other oven. Heat both bars simultaneously, and explain why there is a difference between the two. If you have a sink handy or you can place a basin of clean water on your table, break off a piece of the cooled soap sculpture, and ask a member of the audience to wash her hands with it. Ask if the sculpture still feels like regular soap.