Horsepower isn't about engineering—it's a marketing unit James Watt invented to make steam engines irresistible to mine owners. Rory Sutherland reveals how psychology beats physics: instead of solving problems, make them desirable. From why 16% battery feels terrifying but 56% feels fine, to how Uber tripled the taxi market by fixing anxiety rather than the ride itself, this episode exposes the butterfly effects hiding in plain sight. Inventions only matter when they change behavior; Watt invented 'horsepower' not to describe engines but to let mine owners calculate how many horses they'd eliminate, driving adoption before technical superiority.