1. Give each student a square of heavy cardboard (about 6" ), a ball of clay (about 4" diam.) and a toy dinosaur. They will also need a ring that is made from cutting an 11" x 3" strip from posterboard or similar, overlapping the ends and stapling it together.
2. Each student needs to soften the clay, make a ball, and then press it on their desk to make a thick pancake. They keep turning and pressing it until it is wider that the posterboard ring.
3. The students use the toy to make a deep impression in the clay and then place the clay on the square cardboard. The ring is centered over the clay and pressed all the way down. At this point the students are allowed to peel off and play with the extra outside clay while they wait for their plaster pouring.
4. I set myself up a mixing station by the sink, and mixed batches of plaster in a large plastic container. You need something that holds about 6 cups of plaster. Add water and stir, stir, stir, until it looks like pudding. Pour plaster into each students mold until you think about a generous 1" think layer is formed. Have extra containers to switch to as the plaster will start setting.
5. The plaster needs to dry at least an hour or so. The outer paper ring is then torn off and the clay removed to reveal the plaster cast. After it is really dry, paint with very watery brown paint to make it look like a real rock.