Leave a lasting impression on clay and plaster.
For each "fossilized" item, place a large ball of clay on a piece of waxed paper and flatten it into a thick circle.
Make an impression in the clay by gently pressing in the object and then removing it.
Prepare some plaster of Paris according to the package directions so that it has a smooth but thick consistency.
Spoon plaster into each impression, thoroughly filling it in. Let the plaster dry completely (about 30 to 60 minutes), then peel away the "clay" to reveal the fossil.