Now that coffee is going to be relevant, I promise.
The other week, a friend of mine was telling me about his experiences with the Toyota Prius. The Prius is a hybrid car, which means it has a petrol engine and an electric engine, and it’s got a battery too. He described turning the petrol engine off as he was coasting downhill and charging the battery—getting the energy just from gravity.
The battery’s charge is just shown on an indicator. Was there, he wondered, a better design for this, to encourage people to be more environmentally friendly, by getting them to store gravity as much as possible instead of going downhill on petrol engines.