The simplest periodic body in physics is the pendulum: a weight swinging from a fixed point.

Its period depends only on its length and on gravity — not on how wide it swings, and not on how heavy the weight is. A longer pendulum always swings more slowly than a short one.

This single fact — longer means slower — turns out to govern almost every example in today's session, from a child's leg to a hummingbird's wing.