Subway is the biggest fast-food chain on the planet. The chain knocked down McDonald’s and KFC’s store count decades ago and has more than 44,000 stores in 110 countries. And it all started with a 17-year-old from the Bronx who had never made a sub in his life until opening day. He struggled for years before finding a way to make billions of dollars. And it’s all because of a message scribbled in a notebook.