Entropy

Entropy is a measure of the inefficiency of thermodynamic actions.  You cannot convert all the energy of a thermodynamic process.  It's lost somewhere--heat, friction, noise, radiation, useless motion, etc.  That loss goes under the general term "entropy".  Not only is there entropy in every process, but it's impossible to even measure it directly.  You can only infer that it's there by measuring what you do get back. The laws of thermodynamics, roughly speaking are:
1: Heat can only move from a warm body to a cold one, not the other way.
2: Every thermodynamic process has positive entropy.

Or as my friend Pete Darnell puts it:
You can't get ahead
You can't break even, and
You can't quit the game.
I suspect it's not original to Pete but that's where I first heard it.

One of the consequences of this is that perpetual motion machines are impossible.  If you think you're seeing one, it's because there's something you're missing.  It might be a hidden source of input energy, or it might be that the process is so efficient that it takes longer than your patience to use up the energy it has stored, like a top that can spin for an hour.

I first learned about entropy when at about age 12, I read Isaac Asimov's story "The Last Question", which is about the end of the universe. Entropy is sometimes called "Heat Death" because the losses from things like engines are often manifest as waste heat, but it's really more accurately called "Chaos Death".  It's possible to recover a lot of the heat from various processes and use it for something else--this is sometimes called co-generation.  For example, a blast furnace used in making steel might also power a turbine with its considerable waste heat.   But it's impossible to get it all.   Presuming that the matter and energy in the universe are finite (that's not completely certain), it will eventually all be lost to entropy.  It may take a trillion years, but it will happen.

When I was in my early 20s, I had a very powerful realization, that chaos, including entropy, is a major part of everything that goes on.   This is a good thing, because among things, it gives us a lot of freedom and flexibility, and frankly, fun.  Very little in the world is black and white, not even black and white.  Without chaos, we'd be committed to a predetermined path through life.

<I think this is unfinished but that'll do for now>