Rasp

Rasp briskly rubbed her two rough forepaws together and then pressed them to a young club member's cheek — warm, suddenly, though a moment ago they'd been cool. "Feel that?" she said, delighted. "I didn't light a fire. I didn't plug anything in. I just moved — and warmth appeared. That's my whole primitive: rub two things together and warmth appears. Motion turns into heat. And the deep secret is that the heat didn't come from nowhere. It was in the motion all along."

She rubbed her paws again, slower this time. "Here's the mechanism. When two surfaces slide against each other, they catch — every surface, even a smooth-looking one, is rough and grabby up close. That catching is friction, and friction steals the energy of the motion and hands it to the particles of both surfaces, setting them jiggling harder. Harder jiggling is exactly what heat is — you learned that from Amass. So the smooth, organized energy of my paws sliding gets converted into the scattered jiggling energy of warmth. Motion in; heat out. Nothing is created; the form of the energy just changes."

A young shrew named Tick frowned. "So the warmth is... the motion, wearing a different coat?"

"Exactly that," Rasp said, pleased. "Energy can't be made or destroyed — it only changes form. When you rub your cold hands, the energy of the rubbing becomes the energy of warmth. When a car brakes, all that energy of the rushing car doesn't vanish — friction in the brakes turns it into heat, which is why brakes get scorching. When you bend a paperclip back and forth fast, the bend heats it. When a meteor screams into the air, friction with the air turns its motion into enough heat to make it glow. Everywhere motion is stopped or dragged, the missing motion shows up as heat."