"Yes, like with ice, and with other things."

"Ice is good for that because it doesn't hurt people to touch it."

"Yeah.  It's got less temperature difference to harvest, but the safety would be pretty worth it."

"Less than what, fire?"

"Than the difference between room temperature and absolute zero, when everything stops moving, times two.  For example, ice is zero Celsius - water boils at a hundred - but it's two-hundred-thirty-something degrees in Kelvin, which zeroes its scale at absolute zero, using Celsius's increment."

"- stops moving?"

"Yeah.  On the molecular scale, the thing humans perceive as temperature is motion.  If you've ever seen, like, dust swirling in the air, or how dye diffuses into water...that's a similar thing.

"...There is also a construction of temperature as specifically a measure of entropy, but I couldn't construct the math behind thermodynamic beta if I sat down and tried for a month."

"...I'll take your word for it."

"...where were we, before we wandered off on this tangent?"

"Engines."

"Engines.  Right!  What about them?"

"They can create motion if you have ice, apparently."

"There's quite a lot of ways to get motion from temperature.  Fire's actually a bit more common as a source, because steam's easier to channel."

"What does the steam do?"

"Move through a pipe with stuff in it that turns, then cool back down to go around the pipe again."

"Huh. Does this have anything to do with batteries? I've gotten a few zaps for my trouble so far."

"Electrical engines are often powered by batteries, and the things heat engines drive are very often electricity-generating turbines, so, yes."

"- so you use an engine to make electricity that runs an engine?"

"Sorry, yeah, terminology screw-up on my part.  But often enough you can run a motor backwards to make it function as a generator, what I should have been calling the things that spin turbines and whatnot."

"And the spinning is for....?"

"Do you have mills that grind grain, or - you wouldn't necessarily have power hammers, not with how you can cheat like hell at forging...

"Anyway, they provide a reliable source of steady motion, in a more compact form than donkey-power."

"Yes, mills exist. I think it matters that millstones're heavy? Just spinning won't grind anything."

"You can exchange speed for force - technically, torque - and vice-versa."

"Exchange it how?"

"Gear ratios."