Bonnie Magura (Jackson Middle School, Portland, Oregon) developed this classroom demonstration of lava viscosity. The viscosity of lava that erupts from a volcano controls the shape of the resulting volcano. In turn, the silica (SiO2) content of the erupting lava controls its viscosity, with higher silica (SiO2) content resulting in higher viscosity. (Remember that low viscosity means “runny” lava that can flow down a gentle slope whereas high viscosity means “sticky” lava flows slowly and can form a steep-sided volcano.) By mixing different syrups, a variety of “lavas” with contrasting viscosities can be made. This demonstration invites students to join their teacher as (s)he melts different rocks with different silica (SiO2) contents to form “lavas” with contrasting viscosities. If you are a teacher with a flare for drama, this is your activity!