Layers of the Earth

We are learning about the layers of the Earth in science.  I had the kids make playdough and build a model Earth.  They LOVE the playdough even in 5th grade!
Here is the recipe I used:
2 cups flour
1/2 cup salt
3 T cream of tartar
2 T oil
2 cups water
food coloring

Mix dry ingredients.  Add oil, water, and food coloring.  Mix until only small lumps remain.  Cook over medium heat until dough lumps together.   Be sure to stir constantly.  Kneed untill smooth.  Dough will be sticky and hot at first.  (You can add the food coloring at the end too, if you want a variety of colors)