In 2010, Katrina Lake recruited 20 friends for an experiment: she wanted to see if she could choose clothes for them that accurately matched their style and personality. That idea sparked Stitch Fix, an online personal shopping service that aims to take the guesswork out of shopping. Today, it has about three million customers and brings in more than a billion dollars in annual revenue. PLUS in our postscript "How You Built That," we check back with Justin Li, who created wearable equipment to keep cool and hydrated called IcePlate.