The Launch PadA behind the scenes look at how tomorrow's hottest startups are being primed for greatness Twice a year, in the heart of Silicon Valley, a small investment firm called Y Combinator selects an elite group of young entrepreneurs. Months of intense work culminates in Demo Day, when investors and venture capitalists flock to hear their pitches. Any one of them might turn out to be the next Dropbox (class of 2007), or Airbnb (class of 2009). Randall Stross