Everything we have built so far holds for inertial observers. But what happens when we accelerate, or fall freely in a gravitational field? In 1907 Einstein had what he called “the happiest thought of my life”: gravity is locally indistinguishable from an acceleration. Eight years later this insight became general relativity, in which gravity is not a force but the curved geometry of spacetime. Here we present it qualitatively, with high-school physics.