Lift something and you have banked energy in it. The account balance is $U = mgh$ — read aloud, U equals m g h. $U$ is the gravitational potential energy in joules, $m$ is the mass in kilograms, $h$ is the height in metres, and $g = 9.8\ \mathrm{m/s^2}$ is the acceleration the planet supplies for free. It is not a new idea, either: $mg$ is the weight $F_g$, and $F_g \times h$ is a force times the distance you carried it — the very work you priced in lesson one.

The part students skip, and examiners never do: h is measured from a zero you choose. The floor, the bench, the valley floor, sea level — any of them will do, and none of them is more correct than another. What matters is that you name your zero and then never move it mid-problem. Potential energy has no absolute value; only changes in it are physical, which is why a question that fails to state a reference level is an incomplete question. On this paper the zero is always given to you.