"When?" is the Most Important Question for Entrepreneurs. (The Answer: Now?) When do you launch? When do you act? When do you change? The cofounder of Square has answers.

I am an entrepreneur, but I am also a glassblower, and every glassblower has a mentor.

In fact, we all have the same one: Lino Tagliapietra. Glassblowing is the only profession I know where everyone agrees on who the best practitioner is. Nobody knows who the best accountant or mortician or loan shark is, but the world's best glassblower is Lino.

Everyone learns from the Maestro, usually by meeting someone who has met someone who has taken one of Lino's classes. Maestro's classes are legendary, right down to an admission process that would impress the Harvard registrar. There was even an essay question, and a collection of T-shirts for sale to salve the pain of rejection. It took me 15 years to earn a place, but I was finally admitted.