Most kids learn about the earth's layers at some point during their years in middle school or high school. Their teacher may ask them to make a model of the earth's layers as a homework assignment, and they can do so easily by following the steps provided below.
Paint your small styrofoam ball red and set it aside to dry.
Take four toothpicks and fold a 4-inch section of masking taping in half onto each toothpick. This should result in each toothpick having a 2-inch flag of masking tape on one end.
On your four toothpick masking tape flags, write the words "Inner Core," "Outer Core," "Mantle" and "Crust."
Cover your red styrofoam ball in orange clay, leaving a triangular wedge of the red styrofoam ball exposed.
Cover your orange clay with your yellow clay, leaving the same triangular wedge of red styrofoam exposed, as well as a small wedge of orange clay exposed.
Cover your yellow clay with your blue clay, leaving the wedges of red and orange exposed, as well as a small wedge of yellow; then, you can add chunks of green clay on top of your blue clay to show areas of land on the Earth's surface.
Place your toothpick flags into your Earth so that the "Inner Core" flag is stuck in the red styrofoam, the "Outer Core" flag is stuck in the orange clay, the "Mantle" flag is stuck in the yellow clay and the "Crust" flag is stuck in the blue or green clay.
If you would like a stand for your model of the Earth's layers, then you can use a cup to make your Earth stand up on its own. Simply place the cup right-side up on the table and then set your earth in the cup's opening. If you want to get really detailed with your model, you can add more toothpick flags to your model that explain what each layer of earth is made of.
Play-Doh will not work well for this project as a replacement for clay, because it will dry out, which may cause it to crack and fall off your styrofoam ball.