Every startup starts with an idea.

But how do many entrepreneurs actually come up with those ideas? The reality may surprise you.

We are angel investors who spent the last couple of years interviewing 18 Harvard Business School alumni on their experiences starting companies, for a book called Smart Startups.

For the HBS grads-turned-founders we spoke with, there was no "light-bulb moment" when their business idea struck them. On the contrary, all had embarked on a true ideation process. They vetted ideas often hundreds of them-over weeks, months, or even years.

In our interviews, we identified two types of ideation: organic and deliberate. With organic ideation, the entrepreneur develops an idea based on his or her life experiences or those of family and friends, and builds it over time. With deliberate ideation, the entrepreneur has no specific idea to start with, and instead embarks on a process to find one.

No matter the process, however, our founder interviews revealed that any successful business idea has three factors: a large market opportunity, relevant founder skills, and a huge amount of passion for the project. You might call the fertile ground between these factors "the ideation triangle."

To see how a successful business idea takes root, let's follow one of the founders on his journey: We'll use Matt Salzberg, who graduated from HBS in 2010 and founded meal delivery service Blue Apron in 2012. His company reached a valuation of $2 billion in 2015, went public in 2017, and was just acquired by Wonder Group this past September.

1/ A large opportunity