Explain what it mean to say that energy is conserved

It really means that the total amount of energy in the universe is
a fixed number, and doesn't change.

So if you're watching what's going on in some fixed bottle, box, pot,
or other volume, 'conservation' means that no energy can magically
appear in the bottle or disappear from it.  The total amount of energy
can't change.  The energy can only move from one place to another. 
So if you see the amount of energy change inside there, then you
know that either some of the original energy must have leaked out
and escaped, or else more energy came in from somewhere.

This is actually a very powerful thing to know.  It helps solve a lot
of problems.

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