For many would-be tech startups, Y Combinator is the promised land — an elite pressure cooker where hackers can hone their ideas for 10 weeks surrounded by their own kind. Randall Stross takes a look inside the program, adapting part of his upcoming book The Launch Pad for Vanity Fair. The story takes us from the initial interviews to "Demo Day," showing us what’s going through the heads of young entrpreneurs struggling to come up with the world’s next hit service.