How long will you wait for an elevator? Actually, don’t answer that. Experts, like Otis Elevator Company fellow Theresa Christy, already know.

“Yeah, there is a magic number, and it’s 20 seconds,” she says, “and that’s a number that’s been used for over 50 years in the industry.”

That’s the elevator industry, and if anyone understands our behavior when it comes to riding elevators, it’s Christy. She was the subject of a recent Wall Street Journal feature about her work.

“Traditionally, the wait time is the most important factor,” Christy told the Journal. “The thing people hate the most is waiting.”

She applies her passion for mathematics to making sure people don’t even notice the wait, and for the most part — from the Empire State Building to the 1,483-foot-high Petronas Towers in Malaysia — she’s been successful.

But what about if you bust the 20-second barrier? Christy says she’s heard an urban myth that installing mirrors in the lobby distract people from the frustration of the wait. “Once they’re looking at themselves…they’re not thinking about the fact that the elevator’s a little long in coming.”

For the record, Christy says she’s never used that tactic.

Christy may be good at making elevator systems that run smoothly, but she can’t fix all the awkward situations that happen inside of them. Here’s one of our favorite examples of funny elevator scenes:

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