By 2004, Google had already become well-known for its April Fool's 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 press a while to figure that out, according to Georges Harik, director of Google's in-house startup 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."
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 Fool's 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 startup to the world's largest Internet company by market cap. For the rest of us, it revolutionised not only email but how we thought about cloud storage and Web apps.
Ten years on, Google's rivals have copied Gmail so thoroughly that it's hard to remember just how terrible webmail was before Gmail came along.
Pages were clunky and slow to load, search functions were terrible, and spam was rampant.
You couldn't organise messages by conversation.
Storage capacity was anemic, and if you ran out of space, you had to spend hours deleting old emails or buy more storage from your provider.
Gmail, which was designed using Ajax rather than plain old HTML, taught us that web apps could run as smoothly as desktop applications.
And it taught us the power of cloud storage.
Yet as McCracken explains in his excellent Time story on Gmail's origins, it didn't happen quite the way we all misremember. Gmail is routinely cited as a prime example of a project that sprang from Google's famous employee perk, "20-per cent time," which allowed workers to spend one-fifth of their work week on projects of personal interest.
Not so, Gmail creator Paul Buchheit told McCracken. "It was an official charge" from the start, he said. "I was supposed to build an email thing.
Levy's book backs up that claim. In 2001, Buchheit was removed from product-management in the company's great middle-management purge, and opted to focus instead on building a webmail service. Brin and Page were excited about the idea from the start.
That said, it was never a sure thing that the product would come to fruition: It was initially code-named Project Caribou, a reference to a famous Dilbert boondoggle.