Where is the cloud, really?

Dan has the perfect photo of his dog. His phone says it's “saved to the cloud”. So Dan does what anyone would do: he goes outside and looks up.

Lovely sky. No dog.

Then his phone goes for a swim in a lake. Permanently. He buys a new phone, signs in, and there's the dog, smiling back at him, as if nothing happened. So where was that photo hiding the whole time?

It was in a building

Liz has the answer, and it's a lot less poetic than the sky. The photo was sitting in a data center: a big, plain building packed with computers.

Walk inside and you'd see row after row of tall metal shelves, called racks. Each one holds computers called servers. A server is a computer like yours, minus the screen and keyboard, whose whole job is to store things and send them out when someone asks. They run day and night, because somebody, somewhere, always wants their photos.

When your phone “saves to the cloud”, it sends a copy of the photo over the internet to one of those servers. From then on, one of them is, in Liz's words, babysitting your dog.

What if that building has a bad day?

That's what Dan asked next, and it's a fair question. Buildings flood, lose power, catch fire.

Big cloud services plan for that. Rather than trusting one building, they keep copies of your files in other places. Google, for example, has explained that every action in Gmail is copied to two data centers at once, so if one has a problem, the other carries on. Dan put it better: it's like keeping a spare key at your mom's.

And there are a lot of buildings to spread copies across. Google's own map lists 30 active data center locations in 11 countries, on four continents. Which of them holds your dog photo? You can't tell from your phone, and you don't need to. What matters is that it's on servers, not just in your pocket.

So the cloud is…

Other people's computers. Lots of them.

That sounds less magical than the name, and that's the point. “The cloud” is a friendly word for “stored on a company's servers instead of only on your phone”. The phone in the lake didn't matter, because the photo had a copy somewhere else. And because your account is what connects you to that copy, getting it back is just a matter of signing in.

What the video left out

A minute is short, so a few things didn't make the cut.

Uploading takes time. A photo is only safe in the cloud once it has actually finished uploading. Google Photos, for instance, shows “Backup complete” when it's done, and a big backup over slow Wi-Fi can take a while. If your phone took its swim halfway through, the last few photos may not have made it.

Your account is the key. The photos are tied to the account you were signed in with, not to the phone. Sign in to the same account on a new phone, a tablet or a laptop, and they're there. Sign in to a different account, and they're not, which is a surprisingly common reason people think their photos have vanished.

Private, unless you share. Backed-up photos are private to your account by default. Google's help pages say they're viewable only by you on your signed-in devices, unless you share them.

“Cloud” isn't one company. Lots of companies offer cloud storage. When your phone says “cloud”, it means whichever company's service your phone or app is using, and that company's servers.

So next time your phone says “saved to the cloud”, don't look up. Picture a very large, very boring building, humming away, keeping your dog safe.

What you can do

• Open your photo app's backup settings and check that backup is turned on.
• Look for the “backup complete” (or similar) message before you wipe, sell or replace a phone.
• Know which account your photos are saved to, and keep its password somewhere safe: it's the key to getting them back.
• Sign in to that account on a second device once, just to see your photos there. It's reassuring, and it proves the backup works.