Lift a mass and the work you do is not gone — it is stored, waiting in the height. $U = mgh$ (read aloud: U equals m g h) prices the stored energy: $U$ is that gravitational potential energy in joules, $m$ the mass in kilograms, $h$ the height it was raised through in metres, and $g = 9.8\ \mathrm{m/s^2}$ — the planet's contribution, the same 9.8 that priced weight last chapter.

Keep last chapter's warning in force: $mg$ alone is the WEIGHT, a force in newtons. Stop there and you have not reached an energy — the height still has to be paid. And kilograms are not newtons: a scenario that hands you a mass has quietly left the $\times\,9.8$ to you.