The clay will be the sandstone in our model. The teacher also passes out 3 or 6 sugar cubes, to make different sized caves. The sugar cubes will be the softer limestone in our model. Then they should place the sugar cubes on the clay, so that each cube touches the other, and with at least one cube touching the edge of the clay. Then the students wrap the clay around the sugar cubes, forming a ball.
Students can poke holes in the clay with a tooth pick to simulate porous rock.
Instruct the students to put their cave in the water. The students should observe what happens. Squirt bottles make this more fun, but a simple cup will work as well. That nothing is happening. The cave may bubble for a minute or so until the sugar starts to dissolve.