V is for Volcano.  Using a large triangle for a peak, and two diamonds to represent lava, a volcano can be constructed.  Most children might not need a lot of work to build their understanding of volcanoes, but it is spectacular and mesmerizing to watch videos of magma emerging from volcanoes.  My students and I both enjoy it immensely: 

Model volcanoes from clay are always a fun project too.  You can add baking soda and vinegar for a bang.