Meet $\pi$ properly — the Greek letter pi, said pie, worth about 3.14159. It's the circle's own number: every circle's distance around is π times its distance across, no exceptions, ever. A cylinder is a circle taken for a walk, so its volume stacks the circle floor up the height: $V = \pi r^2 h$ — read aloud: V equals pi r squared h, with $r$ the radius of the circle floor, $h$ the height it stands, and $V$ the volume it holds.

Two habits keep cylinders honest. First, the square touches ONLY the radius — $\pi r^2$ means π × r × r. Second, make sure you're actually holding the radius: problems love to hand you the diameter, the full width, and the radius is half of it. Feed the formula a diameter and the answer comes out four times too proud — the square doubles the damage.