system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Laws of thermodynamics.
gpt
Zeroth law:  Temperature is a thing.  Hot things warm up cold things, eventually.  If you have three things and the first two are the same temperature and the last two are the same temperature, all three are the same temperature.  Nothing terribly profound in this law, but it's important to set some ground rules for a branch of physics that cares so much about temperature.

First law:  Conservation of energy.  You can't create or destroy energy.  If you add heat to a system then that increases its energy.  If you remove heat it decreases the energy.  If a system does work (e.g. a steam engine) then the energy to do that work came from the system, which now has less energy than before it did that work.  If you do work on a system (e.g. compressing the air inside a cylinder) then you're adding that energy to the system.

Second law:  There's this weird concept called "entropy."  In a closed system it can only ever go up (on average, over time).  Entropy can best be thought of as "disorder."  If you have a system that's highly ordered then it can only get more disordered over time.  For example, if you have a tank of air that's sorted such that all the fast-moving molecules are on the right and all the slow-moving ones are on the left then that's fairly ordered.  If you leave this tank to its own devices then the molecules will move around until they're no longer sorted.  This is less ordered.  The tank won't go back to the first state on its own, and if you want to put the tank back the way it was then doing so requires you to mess up some other place to do it.  Note that stars are extremely concentrated locations where there's a lot of very high energy molecules.  It would be more disorder if that energy were spread out across the universe.  Very often when something makes the world more ordered (e.g. an air conditioner making it cool inside and warmer outside) it's by taking advantage of the increasing entropy from the sun radiating heat out into the universe.

Third law:  That "entropy" thing from the 2nd law has some minimal value.  In normal matter molecules are vibrating or moving around.  That's disordered.  It would be more ordered if all the molecules were lined up in perfect rows and not moving at all.  That state is "absolute zero" and is "zero entropy."  In addition to establishing the existence of "absolute zero," the 3rd law also says you can't get there.