Try this! Take two identical cans of soda (or some other beverage). Roll then down a slight incline to confirm that they roll at about the same speed. What do you think will happen if you shake up one can and roll them again? Make a prediction. Then try it. Have your students do it. Develop hypotheses and test them. This is a terrific experiment that anyone can do. What is the best explanation? You can get and read The Physics Teacher or (better) you can come to your own conclusions. David Kagan of California State University-Chico has done a great service to the science education community by describing this intriguing phenomenon.