There has been an interesting development in the lexicon of Silicon Valley the past couple of years — a new way to identify startups that have become really successful. Startups that are valued at a billion dollars or more are called unicorns. There are more than 80 of these startups, according to Fortune, except lately there’s been more and more speculation that  the valuation of some of these companies is about as mythical as unicorns themselves.

Many of these companies have low margins, but the hope is that they’ll build up enough users that they can raise prices. The problem is, they need even more venture capital in order to stay in business long enough to gain those users. As venture capitalists become stingier with who gets their money, some unicorns are finding themselves in trouble. 

Marketplace host Kai Ryssdal spoke with senior tech correspondent Molly Wood about unicorns.

Click the above audio player to hear the interview.