First we mixed the two substances (baking soda and vinegar) in a soda bottle and closed it off with a cork made of a chunk of a potato. Weird? Yes! But the idea was to be able to watch the build up of gasses and bubbles and see the cork blow off, much like some volcanoes have a sort of plug covering their tops. Did you know that? I didn't!! One more reason I love homeschooling--I learn right along with them too. So yes, the cork popped and we tried several different combinations of soda/vinegar to get more of an 'explosion'.
Finally we made our paper mache volcano. FUN, FUN!! We have never done paper mache, so this was a fabulous sticky, glue-y mess. The kids loved it! We let it dry, added more layers, dry some more...Then we let 'er rip! The kids set it up outside in our yard and carefully poured the ingredients in. We all stood back and watched it spew out fizzy, red bubbles. This was a hit! They did it over and over, until the paper started to disintegrate, and the poor volcano we worked on started to look a little worse for the wear.