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A hypothesis is an educated guess about the outcome of an experiment. Discuss the air pockets in the soap and ask your child what they think will happen to the soap when it gets heated. Write down the hypothesis before you begin. Can they think of anything else that expands when heated? For instance, both marshmallows and popcorn expand when you heat them up. Ivory soap is special because it has air whipped into it, but other soaps do not. Perform this same experiment with different types of soap and observe the various results with each brand.  Avoid heating the soap for longer than 1 minute, as it could burn. Record your results and observations underneath your written hypothesis. Talk about what happened with the soap. Ask your child why the soap expanded and why the air pockets are important. Explain to them that there is water trapped in the air pockets and that gas expands when it is heated.  Gases expand because the molecules begin to move quickly when they are heated. This causes them to bounce off of each other and hit the pockets of soap with such force that they expand out.  Discuss the differences between the various brands of soap and the reason you see differences in the results.
Talk about a hypothesis. Experiment with different types of soap. Discuss the results.