Hobbies And Interests
Beach Stones
One experiment that would be well within the capabilities of a third grader would be to try to imitate the erosion you find on a beach. By adding together water, rough rocks and sand in the rock tumbler, and allowing the rock tumbler to run for days or weeks, you can recreate the natural erosion process. Try taking out sample rocks at regular intervals, so you can demonstrate the progression of erosion over time.
Which is Stronger
Another experiment that third graders could do with a rock tumbler is to try and find out which rocks are stronger. A softer rock, such as limestone, might erode quite quickly in a rock tumbler, while something harder, like granite, would take far longer to erode. By tumbling several different kinds of rocks for a set amount of time, you could demonstrate these differences.
Rough to Smooth
A slightly different take on the beach stones experiment would be to use the rock tumbler to smooth down some rocks, without the addition of other materials like sand and water. This would be another, easier way to demonstrate the process of natural erosion. Leave some rocks out of the tumbler, as a "control," to have something to compare the tumbled rocks to.
Make Your Own
If you do not have a rock tumbler, your science experiment could be to make your own rock tumbler. You could easily make a few imitation stones with some plaster of Paris mixture, and then put them in a glass jar with water. You would then shake the jar 100 times, remove a few stones, and then go back to shaking until you have repeated the process five or six times, and have several sets of rocks, each one slightly more eroded then the last.