By now, most of us who use social media will have seen the #2009vs2019 or #10yearchallenge flooding our feeds on Facebook, Twitter and Instagram.
Mostly, it's a bunch of people just sharing carefully chosen old and new photos of themselves.
Some comparisons left us baffled — posts from those who seemingly haven't aged a day left us scratching our heads when they were accompanied by a caption: "what a difference 10 years has made!" What difference?
Then stuff got real.
People starting posting truly dramatic content. And we don't just mean those celebrities who have a few more wrinkles than they did a decade ago.
The English philanthropist and TV presenter was a former model who, in 2008, was the victim of an acid attack that caused major damage to her face. In 2009, she was just 25 and recovering from the brutal, life-altering offense.
Her photo shows her wearing a mask to heal the scars. In the past ten years Piper has become an inspirational role model for many, through her books, TV and charity work. In 2009 she also established the Katie Piper Foundation, which aims to raise awareness of the plight of people living with burns and scars.
She wrote: "Don't let this challenge create anxiety within you about ageing — it's an absolute privilege."
If you were coming of age in the early 2000s then you may well remember the two fictional characters, Jay and Silent Bob. Well, film director Kevin Smith was Silent Bob (and still is, it seems — the Jay and Silent Bob Reboot is currently in pre-production). His transformation, even over the past year, has been incredibly dramatic. Smith suffered a massive heart attack in February 2018 and has since lost more than 20 kilograms. He also adopted a plant-based diet.
His post reads: "Me, myself & I. #2009vs2019 - thanks to @ww [Weight Watchers], going Vegan and Runyon hikes."
The polar bear
This poor little polar bear has been doing the rounds on social media, along with many other dramatic posts that show how much our planet has changed.
The picture of the skinny creature is actually from August 2015, when photographer Kerstin Langenberger posted her work and a lengthy explanation (see below) on Facebook. It has received more than 53,000 shares. The new posts might not technically be a #2009vs2019 difference, but it's the thought that counts.
A coral reef
Planet Love Life, the makers of marine debris awareness bracelets, shared this arresting coral reef shot to raise awareness of what we're doing to our oceans.
"A 10 year photo challenge that REALLY matters," says the post.
"Coral reefs are dying at an alarming rate all around the globe. They need our help. We are dedicated to saving ALL life on our planet, including corals."
In that picture you see a beach on the French Caribbean island of Guadeloupe (where the British-French drama series Death in Paradise was filmed). Yet more proof of what we and global warming are doing to our planet.
Glacier number one
These photos weren't taken in 2009 and 2019, either. Nor is it a difference of 10 years (it's 11). This is the Trift Glacier in Switzerland circa 2006 and 2017. It again isn't exactly playing by the rules of the challenge, but when they're trying to highlight concerning environmental changes, we'll take it.
Glacier number two
Just in case one melting glacier comparison isn't enough to convince you, we thought we'd share this stunning post by Dr Kristen Weiss, a scientist, conservationist and self-proclaimed ocean-lover. She shared two pictures of the Mendenhall Glacier in Alaska, which, she says, has retreated by 1/3 mile in less than a decade due to a warming climate.
"The world's glaciers are melting at alarming rates," she wrote on Twitter. "We're losing freshwater fast, along with permafrost and precious habitats."
The ominous theory
You may have heard it, but perhaps you didn't want to think about it. Well, author Kate O'Neill said it — and nearly 20,000 people liked it. Could all the images posted for this challenge be used in ominous ways in the future?
"Me now: ponders how all this data could be mined to train facial recognition algorithms on age progression and age recognition," she wrote.
It could sound like something straight out of a Black Mirror episode, but is there any truth to it?
People naturally rebutted the claim, arguing Facebook already has all those profile pictures users have been posting since it launched in 2004. But O'Neill gently rebuts the rebuttals.
It made you think, didn't it?