Mayer in her Google days.
Almost since its beginning, Google has had a well-known policy called "20% time."
Under this policy, employees get to work on whatever personal projects they want during one-fifth of their week - a whole day.
Supposedly, lots of big Google products came out of 20% time. One you've definitely heard of is Gmail.
20% time is considered such a successful policy at Google that policies like it have spread to several other companies in Silicon Valley, from LinkedIn to Google.
But here's the thing.
It turns out 20% time doesn't really exist at Google. Not according to early Googler and Yahoo CEO Marissa Mayer.
While doing reporting for my new book - "Marissa Mayer and the Fight to Save Yahoo!" - I learned that in the Spring of 2013, Mayer stood up on stage during an all-employee meeting at Yahoo and debunked the 20% time myth.
Mayer was announcing something called the CEO challenge - an initiative where teams that came up with cool new product ideas would get spot bonuses of $250,000. Mayer warned Yahoo employees not to work on CEO challenge products instead of doing their regular work.
"It's funny," she said, "People have been asking me since I got here when is Yahoo going to have 20% time."
"I've got to tell you the dirty little secret of Google's 20% time. It's really 120% time."
She said that 20% time projects aren't projects you can do instead of doing your regular job for a whole day every week. It's "stuff that you've got to do beyond your regular job."
If you're the kind of person who likes these sorts of inside details, you're the kind of person I wrote my book for, and you should buy a copy - or three dozen for all your friends.