If not, the premise is simple - post a 2009 photo of yourself next to a recent one, to show how much you've changed.

Millions have taken part, but some have criticised it for being - among other things - narcissistic, ageist and sometimes a bit sexist.

But now people are using the hashtag to reflect on bigger changes. Such as...

In this tweet, footballer Mesut Ozil compares what appears to be a large iceberg on the left, and a melted iceberg on the right.

Although the meme isn't entirely accurate - the photo on the left, of the Getz Ice Shelf in Antarctica, was taken in November 2016 rather than in 2008 - there's no denying that shrinking ice sheets continue to be a major issue.

According to Nasa, Antarctica is losing about 127 gigatonnes of ice mass every year, while Greenland loses 286 gigatonnes annually.

This, they say, is largely down to rising global temperatures being absorbed by oceans. The planet's average surface temperature has risen by about 0.9C since the late 19th century - and about a third of that has happened in the last decade.

Environmental activist groups are also using the hashtag to highlight this issue.

This post from Greenpeace bends the challenge a bit, by comparing archival photos from 1928 with images taken by Swedish photographer Christian Aslund in 2002.

And Martin Kobler, the German Ambassador to Pakistan, tweeted out an article about climate change in the Pakistani region of Balochistan.

According to the latest Global Climate Risk Index, Pakistan has been the eighth most-affected by climate change over the last two decades.

The year 2018 was when the world really woke up to the reality of plastic pollution as well as climate change.

Scientists calculate that about 10m tonnes of plastic waste end up in the oceans every year - and that some of that can take hundreds of years to biodegrade.

So some campaigners are turning the #10YearChallenge on its head to show that, while we might have changed a lot in the last decade, the plastic we throw away remains almost exactly the same.

On 17 December, 2010, a Tunisian street vendor called Mohamed Bouazizi refused to pay a bribe to local officials, so had his fruit and vegetable cart confiscated as a result. Faced with an unforgiving bureaucratic process, he set himself on fire.

This act, less than 10 years ago, was the catalyst for what was later known as the Arab Spring - a wave of protests across the Middle East and North Africa that, in some cases, led to bloody civil wars and a refugee crisis that saw a record number of people forced from their homes.

To reflect on this, then-and-now photos of conflicts in Syria, Libya and Iraq have been posted on Twitter - with thriving street scenes contrasted with stark images of crumbling buildings.

People are also posting photos of Yemen, where a three-year civil war has led to the world's worst humanitarian crisis today.

And some positive changes, too

People are also sharing some of the things that appear to have improved over the last decade.

According to statistics from the World Bank and the UN, extreme poverty is currently at its lowest overall level in recorded history, child mortality and youth illiteracy are both down, and global average life expectancy has increased.

But, sadly, this doesn't show the whole picture.

While extreme poverty is at its lowest level globally, it is still getting rapidly worse in Sub-Saharan Africa - where the average extreme poverty rate is now about 41%.

And although youth literacy has increased overall, it's still lowest in less developed countries - and it affects young women more than men. According to the most recent data, 59% of all illiterate youth are girls.

While environmental damage is still very much an issue, there's no doubting that awareness of it has increased, leading many people and countries to try and switch to sustainable sources of energy.

The International Energy Agency says that renewables surged in 2016, thanks in no small part to a boom in solar panel installation in China.