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Program Sea Monsters: A Prehistoric Adventure

  • 1. Look at the illustrations to see one way a fossil forms.
    Look at the color illustrations and read each caption to learn how a fossil forms. 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. Look at the black-and-white illustration of what a mold and cast look like. You're going to make a mold and a cast of an object!

    2. Choose an object and make a mold.
    Shape some clay into a small circle slightly bigger than the object you selected. Place the circle on a flat, dry surface. Make a rim around the top edge.

    3. Use your object to make an imprint.

    Smooth a small amount of vegetable oil on the object so it will not stick to the clay. Carefully push the object into the clay. Then remove it. The imprint left behind creates a mold.

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

    5. Compare your cast to the original object.
    Allow the cast to dry for 30 minutes or more until it is cool and hard. Carefully separate the cast from the mold. How does it compare to the original object?

  • Materials You Provide

    • A small, solid object such as a shell, coin, key
    • Clear cups or bowls
    • Modeling clay
    • Plaster of Paris
    • Stir sticks
    • Vegetable oil
    • Water

    Required Technology

    • Internet Access: Optional
    • Tech Setup: 1 computer per classroom, Projector

    Recommended Prior Activities

    • None
  • Background Information

    Scientists sometimes find casts of fossils. Casts are copies of the original object.


    Term Part of Speech Definition Encyclopedic Entry
    cast Noun

    impression formed when a liquid substance is poured into a form or mold, and then hardens into that shape.

    fossil Noun

    remnant, impression, or trace of an ancient organism.

    Encyclopedic Entry: fossil
    mold Noun

    hollow structure used to give form to a liquid substance as it hardens.


National Science Foundation