Thiel says it's easy to figure out why someone wants to be a CEO or another very early employee in a startup; they'd like to run a company and get rich doing it.

He says its also easy to know why employee number 1,000 joins; the company is clearly on its way to growing into something huge, and will provide a nice, stable living.

Employee number 20, he says, will have to join for different reasons.

By then, the big equity stakes will have already been handed out.

Also, a company with only 19 employees won't have "made it" yet; it won't be a place someone looking for a stable income will join. So what's the right answer?

According to Thiel: for the only companies worth starting – perhaps the only companies he'd invest in – the right answer is that employee number 20 will join because you are doing something nobody else has done – "something fundamentally new, fundamentally different."

Via: http://www.businessinsider.com/the-most-important-question-peter-thiel-asks-any-startup-looking-for-money-2011-7