I always thought a yard was three feet, then I started mowing the lawn.

The line is a compact joke built on a pun: a “yard” is both a unit of length (three feet) and a lawn/grounds around a house. The speaker claims to have understood the measurement definition until the practical reality of maintaining a “yard” (mowing) revealed how large and labor-intensive it feels. The humor comes from the sudden shift from abstract knowledge to embodied experience—what seems simple in theory becomes daunting in practice. As a proverb-like quip, it also gestures at a broader theme: lived work recalibrates our sense of scale, effort, and time more than textbook definitions do.