In 2004, two former Paypal engineers, Jeremy Stoppelman and Russ Simmons, were spit-balling new internet ideas. Out of their brainstorm came a site where you would email your friends asking for local business recommendations. The launch was a flop, but they discovered that people seemed to enjoy writing reviews not just for friends, but for the general public. Fifteen years later, Yelp is a publicly traded company with more than 4,000 employees and over 140 million monthly visitors. PLUS in our postscript "How You Built That," Liz Bales explains how putting cat food inside plastic mice became her full-time business and why it could revolutionize the way humans feed their cats.