Just before kids are ready to use plaster of paris: For each child, mix half cup of plaster of paris with two and a half tablespoons of cold water in a paper cup. A one-fourth teaspoon of salt speeds hardening.
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Students’ imprint fossils
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Plaster of paris (at art and hardware stores)
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Salad oil, small jar
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1 half-pint milk carton per child, with top cut off
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Paper cups
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Plastic spoons
Ask: What might happen if you poured something that could harden into your imprint fossil?
Distribute milk cartons and spoons and have students wipe clay imprints lightly with oil.
Kids place imprints into milk cartons and, using spoons, cover them with about an inch of plaster of paris.
Leave the cartons in a warm, dry spot.
When the plaster of paris feels very hard (after 30-60 minutes), peel the milk carton away and gently pull off the clay. The hardened plaster of paris leaves cast fossils. Ask: Which type of fossil is most like the original object? Why?
Have the students exchange cast fossils. Can they figure out what objects were used to make casts? How?
Go on a search for animal prints. (You can also have children look for human footprints and tire tracks.) Look at cement, mud, and snow. Ask: Which animals or objects made which tracks? How can you tell?
