Electric Play Dough 4+
There's not much I can say here, because the blog world already has us covered with great demos for making electric playdough. See this video for a great demo.
The electricity conducting dough is made using this recipe. Squishy circuits also provides the recipe for insulating dough.
We made lamps with every kind of play dough. Saturday science!
The kids in my class tasted their dough to make further hypotheses about what types of materials are conducting (awesome!), and instead of battery packs we just stuck our batteries together in series.