Elisha Otis did not invent the elevator, but he invented the safety brake that made skyscrapers possible. Before Otis, elevators were considered dangerous; if the rope snapped, the car plummeted. Otis designed a system with a spring that would engage metal 'dogs' into notched guard rails if the tension in the cable was lost. He famously demonstrated this at the 1854 World's Fair in New York by cutting his own elevator cable while standing inside. The safety brake gave people the confidence to ride in elevators, allowing architects to build vertically rather than horizontally. This invention is the primary reason modern cities look the way they do, as it enabled the construction of high-rise buildings and the concentration of urban populations.