Roll one color of clay into a ball about 1 inch in diameter, either in your palms or against a hard surface, to form the solid inner core.
Flatten a second color of clay with a roller or rolling pin into sheets approximately 1/4-inch thick.
Wrap the clay sheets around the ball of clay until the ball has roughly doubled in size. The clay sheet represents the second layer, the liquid core.
Smooth the surface of the second layer by rolling it gently on a hard surface.
Flatten a third color of clay with a roller and apply a new layer to the model in the same manner to form the mantle. The ball should now be about 3 inches in diameter.
Flatten a fourth color of clay into a much thinner sheet to represent the crust and apply it to the model.
Slice the model in half with a sharp knife, using gentle pressure, to expose the four layers. This should be done by an adult.