While teaching, I used to demonstrate what bothered Leibniz to my students by dropping my wallet. I would ask them what caused the wallet to fall. They would answer “gravity” or “gravitational force” without thinking too much about their answer. I would then ask them, “But what is gravity?” They would typically think about the question for a bit and eventually (sometimes with some prompting) come up with answers such as “the force that causes things to fall” or “the tendency for unsupported objects to fall.”
