The number one interview question, according to this CEO and former Googler

At the online bra company ThirdLove, curiosity is one of the most important company values and something every new hire has to display.

But flat-out asking a job candidate if they're a curious person is hardly the best way to find out whether they really are. (Who would say no?)

Heidi Zak, a former Google senior marketing manager and a cofounder — along with her husband, Dave Spector — of ThirdLove, asks every job candidate this question to assess their level of curiosity:

"One year from now, if you're part of the team at ThirdLove, how will you judge if [your time here] has been a success?"

The ideal answer, she says, would start off with something like, "It's all about learning new things and about growing professionally."

Zak and Spector founded ThirdLove in 2013. Since then, the company has raised over $13 million in funding, according to Crunchbase, and grown to 50 employees. Zak said hiring has been one of the hardest parts of building the company.

To make sure it didn't bring on anyone who wasn't a personality fit, ThirdLove's leadership team identified a set of core company values. Beyond curiosity, those values include being positive, responsible, adaptive, and able to put the customer first.

Then they developed specific questions — like the one mentioned above — to see whether candidates "inherently possess those values in their personality," Zak said.

Ra'el Cohen, ThirdLove's vice president of design and product development, shared another question the team uses to assess personality fit: "What was the last mistake that you and your last team made, and what did you learn from it?"

They're looking for the candidate to use the word "I" instead of "we" — to take ownership and responsibility for the mess-up instead of blaming it on others.

"That's a real telltale sign," Cohen said.

"In a startup, you need to own your own world," Zak added. "If they don't do that, then that's very much a red flag."