Resonance

push the swing till her foot rings the bell.

push a swing in time with its rhythm, a chronophotograph of every swing, to ring a bell; pushing out of time kills it

A swing has its own rhythm, set by its rope. A push adds to it only when it goes the way the swing is already going, so the pushes that build it come once a swing, just as it starts away from you; push while it is coming back and you take away what it had, and pushing all the time builds nothing. Small pushes in time with a thing's own rhythm add up: that is resonance, why soldiers break step on a bridge and why a held note can set a wine glass ringing. A heavier rider swings to the same rhythm and needs more pushes; a shorter rope swings to a quicker one.

Concepts: resonance, timing.

An interactive you play with one thumb, on Wundr.