Make a Volcano!! baking soda, vinegar & dishsoap (the dishsoap really helps it foam more!) You can use a volcano to talk about what you put inside yourself, eventually comes out, (ie...music, bad language, movies, etc) keeping your tongue from evil, explaining how we should be SLOW to speak, therefore avoiding words that hurt people.
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Our oldest son, who's in 3rd grade, brought home the classic "build a volcano and make it explode" class project. We started with several items that were essentially recycled: an empty soda bottle, a leftover piece of plywood for the base, a cardboard box and our old phone book. In class, the volcano was subjected to the classic baking soda and vinegar experiment to simulate an explosion. Our son told us that his volcano had the most powerful explosion in the class.