Marcel worked under my supervision to solve a problem involving the conservation of momentum. He had to realize that a system can start or end in zero total momentum even when objects are moving. He was able to recognize how both sides of a conservation of momentum equation are equal to zero with the initial conditions having no moving objects and the final conditions involved two moving objects. I then asked him to solve a thought experiment involving a person walking on a raft in still water. I finished the class by explaining the difference between elastic and inelastic collisions. There was no assignment.