A student returning from winter break relayed a conversation he had with his father about garage doors. The father wondered why a garage door would stay in place after being pulled down because of the strong pull of the stretched spring. The student showed me the explanation he had come up with and it turned out to be spot on. He had not only determined that different weight doors might need springs of different stiffness, he was excited to find that when he went to Home Depot to check, that the springs came in a couple of different stiffnesses marked for the weight of the door. He was also able to determine that the door would stay in any position it was lowered to - quite a realization for a newbie to physics.
Good to see that some of what we did last semester stuck.