I recently went to the park with my kids. While at the park, my daughter built a “rock castle”, which is just large pile of rocks. My assignment, given to me by my three year-old, was to drop rocks on the top of the pile, one at a time. I did. And I noticed something. Sometimes a rock that I dropped would bounce right off of the top and dribble its way down the pile. I could drop another rock in that exact same spot and it would land right on top of the pile, supported by the rocks around it. I could take that rock that bounced, and drop it in one of the other piles created by my daughter, and it would stick.