Last day of doing this daily! After today I’ll settle down to weekly like I planned.

Friction between pieces of sandpaper has an obvious source: Sandpaper is lumpy. But lumps and bumps, whether at centimeter or atomic scales, are the source of any friction between two surfaces that are rubbed together. When bumps of one encounter lumps of another, they catch. (Everything is also made of electric charges that fleetingly attract the charges of other surfaces. On really big scales, this sort of process helps make lightning. But that’s a story for another time.)

Overcoming friction means shoving atoms out of the way, jiggling nearby atoms in the process. So friction turns useful energy into useless energy—what physicists call entropy. Those two sentences say the same thing, by the way. One just focuses on atoms while the other is bigger-picture. But I think it’s easiest to understand friction if you switch zoom level mid-sentence: Friction turns kinetic energy (energy of large-scale movement) into heat (energy of atomic movement). The more heat you get, the less kinetic energy you keep—and kinetic energy is the energy we use to power things like engines.

Energy is constantly changing forms. That’s all it can do, dark energy aside. It can’t appear or disappear. But as we’ve already seen, some energy is useful and some isn’t. The second law of thermodynamics says that over time, we’ll run low on the former while accumulating the latter.

The universe, then, is like a cashier who only gives change in pennies. Sure, you might pay for something with exact change (you might find some process that turns 100% of useful energy into other useful energy), but honestly that’s pretty rare and never really happens if you have a complex bill. The rest of the time, you put in more energy (more money) than you need, and the change you get back is completely useless.

Admittedly, we should probably try to pay in exact change during a coin shortage. I’m sure there’s a metaphor for renewable energy in there somewhere.