Cold isn't a thing that flows — heat is. It's tiny jiggling, and it always slides ONE way: from warm into cold. The warm hand's heat flows into the ice pop.
See the rule for yourself: tap — the heat dots always drift toward the cool side, never backward. But then how can the AC push heat OUT of your cool room into the HOTTER yard? It needs a trick…
Here's the trick: SQUEEZE a gas and it jiggles harder — HOT, like a bike pump! Let it EXPAND — icy, like spray from a can. Now the AC can make a pipe COLDER than your room, or HOTTER than the yard.
Squeeze = hotter jiggle. Expand = colder calm. Same gas, both ways.
The loop uses the rule TWICE. Expanded, the pipe is icy (5°) — colder than your room — so room heat falls IN and rides along. Outside, the SQUEEZE crams it hot (60°) — hotter than the yard — so it falls OUT. Downhill both times!
Your turn: the room is a hot 30°. Run the AC and watch the room's number FALL — its heat is riding the loop out, downhill twice. Slide back to off: the hot day sneaks it right back up. The box outside blows HOT the whole time.
room 30° — a hot day
Summer: solved. But WINTER flips the problem — heat sneaks OUT of your house by itself, so you need heat coming IN. Fix one: the FURNACE just MAKES heat — a careful campfire in a box. But making heat is hungry: every bit of warm costs fuel.
A furnace makes heat. But making it costs a lot of energy…
But option two is sneakier…
Fix two is sneakier: don't make heat — CARRY it in, the way the AC carries it out. Just run the squeeze-loop BACKWARDS! But hold on… outside is freezing. Is there any heat in 0° air to carry? 🤔
It's a freezing 0° day. We flip the loop backwards.
What happens to the house?
Slide to flip the loop 🔀
