Rasp and Smooth

Rasp went first, sliding her rough paws together until they warmed. "My half is instant. Take neat, organized motion — my paws sliding, a car rushing, a drill spinning — and friction converts it into scattered heat in a heartbeat. Motion in, warmth out, no effort at all. It's the easiest thing in the world; it happens whether you want it or not." She pressed her warm paws to Ember's cheek. "See? A flick of motion, and it's already heat. Going down the street — from tidy motion to scattered warmth — is effortless and instant."

"And my half," said Smooth, "is the cruel twist: you can never run back up. Rasp scattered that motion into heat in an instant — but the heat will not gather itself back into neat motion, not ever, not on its own. The warmth in Ember's cheek will spread out into the room toward equilibrium and simply dissipate. It will not spontaneously collect itself, leap back into Rasp's paws, and set them sliding again. Scattering is easy and instant. Un-scattering, on its own, essentially never happens — because there are countless ways for energy to be spread out and almost none for it to gather back concentrated."

Ember's eyes widened. "So Rasp can turn motion into heat in a snap — but nothing can turn that heat back into the same motion for free?" "That's the one-way street," said Smooth. "It's why a car's brakes can turn its rushing motion into scorching heat, but that heat can never push the car back up to speed by itself. Why a bouncing ball's motion leaks into warmth with every bounce until it stops — and a stopped ball never gathers warmth from the floor to start bouncing again. Every real machine leaks some tidy energy into scattered heat, and that heat never comes all the way back. Energy is never destroyed — but it slides, one way, from useful-and-organized toward scattered-and-useless."