This took about 20-25 minutes to complete and started out with them cutting out the phases of the moon from a piece of paper. They then had to arrange them in order and show that the phases are a cycle.
Then I passed out the oreos and demonstrated how to remove the frosting with a spoon. This came easier to some groups more than others. (They were not graded on how well they were able to remove the frosting, rather that they had the phases in the correct order and had removed the appropriate amount of frosting.)
I had some very happy third graders when they were finished as they were allowed to sample the phases :)