Square Perimeter

Four equal sides, so the distance around is simply four times one of them. The formula earns its keep in the fencing aisle: 100 ft of fencing encloses a square 25 ft on a side, covering 625 ft² — and no other rectangle made from that same 100 ft does better. That is the isoperimetric principle in miniature: among all rectangles of a given perimeter, the square holds the most area, and among all shapes whatsoever, the circle does.

Comparing $P = 4s$ with $A = s^2$ also shows why big things are cheaper to wrap than small ones. Double the side and the perimeter doubles, but the area quadruples — the reason bulk packaging uses less cardboard per litre, and the reason a small animal loses body heat faster than a large one.