A neutron star is the endpoint of stellar evolution after a supernova explosion. The remnant compresses so much that a teaspoon of its material weighs billions of tons. Fritz Zwicky predicted this in the 1930s, and Jocelyn Bell Burnell later discovered them as pulsars. The sound inside, a pressure wave, races at nearly the speed of light. But it can't outrun light. To keep the star from collapsing under gravity, the sound wave must make it from the center to the edge. It's like a relay race with a strict speed limit on the runners: the lap can't be too big, or the relay won't finish. Hence a simple rule: for a heavier star, the radius can't exceed the radius of a lighter one by more than 1.16 times minus a kilometer. Data from the NICER telescope confirmed: for the most massive pulsar, the radius shrank from 12.5 to 11.4 km. In the future, this limit will weed out errors and point to dark matter in the cores. Gravitational waves from mergers, caught by LIGO, will also be tested with this rule. Because of the monstrous density, spacetime around is curved, and the star acts as a gravitational lens.