eli5: Why do cold things get hot, and warn things get cold?
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The universe strives for a state of entropy (I say strives, but it doesn’t have a goal, really – it just moves in that direction naturally). This means an even distribution of all matter and energy across the universe. Some fundamental forces combat this, but the trend toward entropy is relentless.
Heat is a concentration of energy in a specific object and entropy wants to stop that – it wants that energy evenly spread out across everything. So the hot thing will have it energy radiate out and warm its surroundings until they are all the same temperature (and even spread of the heat energy). Conversely, a cold thing will have energy radiated into it, cooling its surroundings until they are all the same temperature.
It’s the 2nd law of thermodynamics.
It says basically that heat will always travel from the hot to cold.
So if you have something hot- a pie for example, it will cool as heat flows from the hot pie to the surrounding area.
If you have a frozen pie, heat will travel from the warmer surrounding environment and will warm the pie.
Temperature is closely linked to the motion of atoms in an object: They always move or wiggle around a bit, randomly bumping into each other. In a cold object they move slower, in a hot object they move faster. What happens if the two touch each other? Some fast atoms will collide with slow atoms and transfer some of their energy, just like a fast football (or golf ball, baseball, …) can speed up another football it hits. Wait a while and the energy gets spread evenly: The cold object heated up and the hot object cooled down.
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