Two different questions live on every box: how much does it HOLD (volume, in cubes) and how much does it take to COVER it (surface area, in squares — paint, wrapping paper, cardboard). This lesson is the covering. A box has six faces, and they arrive in matching pairs: top and bottom, front and back, side and side. Add one of each pair, then double the lot: $S = 2(lw + lh + wh)$ (read aloud: S equals two times, l-w plus l-h plus w-h): $S$ is the surface area, and $l$, $w$, $h$ are the very same length, width and height that gave the volume.

The classic slip is counting each face once — that pays for the top and forgets the bottom, and half the box goes bare. On a cube the pairs collapse into one happy number: six identical squares, $S = 6a^2$ — six a squared. And watch the units tattle: surface answers wear m², never m³ — if your units come out cubed, you've answered the filling question by mistake.