By 2004, Google had already become well-known for its April Fools' pranks. That year, it appeared to up the ante, with not just one but two suspiciously improbable announcements. The first was a posting for job openings on the moon. The second was that the search company was launching a free email service that would offer users, not one or two megabytes, but 1000 megabytes of storage. If that were true, it would mean the average user would never have to delete an email again.

The joke, of course, is that it wasn't a joke at all. Gmail was a real product, and it really did offer multiple orders of magnitude more storage than Hotmail and Yahoo Mail, the leading competitors at the time. But it took the tech media a while to figure that out, according to Georges Harik, director of Google's in-house start-up incubator at the time. "Journalists would call us and say, 'We need to know if you're just kidding, or if this is real,'" Harik recalled to Time's Harry McCracken. "That was fun."

Google thought it would be funny to launch a free email service on April 1, 2004.

In most people's eyes, it was actually a PR blunder, reported Steven Levy in his 2011 book In the Plex. But Google co-founder Sergey Brin never saw it that way. "Even years later, Brin still relished the reverse spin – tricking people by not hoaxing," Levy wrote. Early Gmail product manager Brian Rakowski also remembers Brin being tickled by the move. "The ultimate April Fools' joke was to launch something kind of crazy on April 1st and have it still exist on April 2nd," Rakowski told McCracken.

Hilarious? Well, that depends on whether you share Brin's lopsided sense of humour. But if introducing Gmail wasn't the funniest thing Google has ever done on April 1, it was certainly the most important. For Google, it was a turning point on the path from a scrappy web-search start-up to the world's largest internet company by market capitalisation. For the rest of us, it revolutionised not only email but how we thought about cloud storage and web apps.