Circles bring one number and one trap. The number is $\pi$ — the distance around any circle divided by the distance across it, the same for every circle ever drawn. On this paper, $\pi = 3.14$. Space inside: $A = \pi r^2$ (A equals pi r squared). Distance around: $C = 2 \pi r$ (C equals two pi r) — that one is the circle's perimeter wearing its own name, the circumference.

The trap: both formulas want the radius, centre to edge — but problems love to hand you the diameter, edge to edge through the middle. $r = \tfrac{d}{2}$; always halve first. Feed a diameter into $\pi r^2$ unhalved and the square doubles your doubling: the answer lands FOUR times too big. That slip never retires — meet it now, on purpose.