Quoting Jen K.:" does it have to be made out of clay? How about paper mache? http://www.toppartyideas.com/science-experiment-volcano-explosion/"
Quoting Holly and girls!:" It doesn't HAVE to be made out of clay, it can be made of anything really, but paper mache seemed like more work than clay. ( I am the most un-crafty person on the planet)"paper mache is just watered down glue, and you dip strips of newspaper in it.