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why does heat flow from hot to cold
Leave a metal spoon in hot tea and soon the handle warms your fingers. Heat always runs hot to cold, never backwards. Why does it have a direction?
First, a correction. Nothing called cold flows in. Heat is energy on the move, and cold just means less energy to give away.
Zoom into matter. Every atom jiggles, and temperature is how hard they jiggle on average. A hot block has fast jigglers, a cold block slow ones.
Where the blocks touch, fast atoms bump slow ones. On average a bump hands energy to the slower atom, so the slow side picks up energy.
Bumps never stop, so the trickle keeps going until both sides jiggle alike. At equal temperature the energy is shared evenly and the net flow stops.
Put it together. Hot atoms jiggle hard, bump their slower neighbours, and the energy spreads along the spoon until your hand feels it. Heat runs hot to cold.
Heat has a direction because jiggling atoms share their energy by bumping, and sharing evenly is overwhelmingly likelier than un-sharing.