Bob Beamon landed so far down the pit in Mexico City that the optical device built to measure the jump couldn't reach his mark. Officials went looking for a steel tape. It read 8.90 meters, and nobody matched it for nearly twenty-three years.

The crowd in the stadium trusted their eyes, and so did everyone watching on television. They had seen a world record, the kind of feat that comes once in a generation, and its rarity only made it more remarkable.

Had that jump been a laboratory result, scientists would have raised an eyebrow. A finding that appears once and never again usually points to a faulty instrument or a hopeful researcher, so science asks for repeatability before it believes anything. The rule keeps researchers honest, and it has earned its place.