While speaking at the Mobile World Congress in Barcelona this week, Facebook CEO Mark Zuckerberg revealed his one main rule for hiring employees.

I will only hire someone to work directly for me if I would work for that person,” Zuckerberg said, according to the Telegraph. “It’s a pretty good test and I think this rule has served me well.”

During a Q&A session at the MWC, Zuckerberg told the audience that Facebook isn’t a company for everyone, so he looks for new hires whose values align with the company’s. He also said that Facebook’s relatively small (in comparison with other tech companies like Google) staff is a “crucial” part of its success, according to the Telegraph.

The most important thing is to keep your team as small as possible,” he said. “[Facebook] serves more than a billion people around the world but our team has fewer than 10,000 people. It’s only possible because of modern technology. Big companies get bloated.”

In addition to sharing his views on hiring, he also had some advice for entrepreneurs, saying that the most important thing for them is to have faith in themselves and trust themselves.

“When you’re young you hear that you don’t have experience to do things, that there are people that have more experience than you. I started Facebook when I was 19,” he said. “Don’t discount yourself, no matter what you’re doing. Everyone has a unique perspective that they can bring to the world.”

Watch a replay of Mark Zuckerberg’s keynote appearance at Mobile World Congress 2015 below: