Facebook's hottest product was inspired by a newspaper in 'Harry Potter'

Facebook's video business is hot stuff right now. Users are watching 4 billion videos a day - shooting up from just 1 billion in September 2014.

And the key feature driving that growth was inspired by teen fantasy series "Harry Potter," Facebook chief product officer Chris Cox has revealed.

In an interview with Fortune's Erin Griffith, Cox discusses the early days of video on the social network.

Engineers were struggling to get users to actually click on and watch videos - and then, "one day, the team remembered the 'Daily Prophet' newspaper from the Harry Potter movies."

Here's how the Daily Prophet looks:

As seen above, the Daily Prophet's images are magical, with their subjects moving around on the page - more like video than a traditional photograph. The engineering team drew inspiration from this. "News Feed should clearly be alive and move in your hands on smartphones," Cox told Fortune - and Daily Prophet-style autoplay was the obvious way to achieve this.

In December 2013, autoplay was added - and views skyrocketed, transforming Facebook into the video behemoth it is today. Elsewhere in the Fortune story it's revealed that Facebook enjoyed 10 billion views of "Ice Bucket Challenge" videos - that's 10 times as many as on YouTube.