1. Display the illustrations to show students one way a fossil forms.
Scroll through the color illustrations and read aloud each caption to help students understand how a fossil forms. Explain to students that fossils form in many ways. Sometimes a leaf, shell, or foot leaves an imprint in soft earth. When the imprint hardens, it forms a mold. Later, mud or other materials can fill the mold to make a cast—a copy of the original. Have students look at the black-and-white illustration of what a mold and cast look like. Tell them they will make a mold and a cast of an object during this activity.

2. Have students choose an object and make a mold.
Provide students with some clay. Have each student shape the clay into a small circle slightly bigger than the object they selected. Have them place the circle on a flat, dry surface and make a rim around the top edge.

3. Have students use their selected object to make an imprint.
Have students smooth a small amount of vegetable oil on the object so it will not stick to the clay. Then ask them to carefully push the object into the clay and then remove it. Provide assistance, as needed. The imprint left behind will create a mold.

4. Help students make a cast.
Follow the mixing directions on the Plaster of Paris package. Quickly pour the mixture on students' clay models. Fill the mold to the rim. Tap and shake it to remove air bubbles.

5. Have students compare their casts to the original objects.
Allow the cast to dry for 30 minutes or more until it is cool and hard. Help students to carefully separate the cast from the mold. Then have a whole-class discussion. Ask: How does it compare to the original object?