Have you suddenly noticed black-and-white portraits in your Instagram feed?
They'll likely be closeups of women's faces — some famous, plenty not — with a few names tagged and a caption that declares "#challengeaccepted".
So, what is the challenge exactly?
Damn good question.
The social media trend professes to be an exercise in #womensupportingwomen, in which we invite each other to upload a grey-scale selfie in an apparent show of solidarity and celebration of the sisterhood.
Why this particular medium was chosen remains a mystery. As do the challenge's exact origins.
What is #ChallengeAccepted about?
It appears early posts using the formula were made by Brazilian women, some time around the middle of July.
Local media outlets there were reporting at the time that celebrities had begun sharing selfies with little context beyond the phrase "desafio aceito" (challenge accepted).
According to those reports, the posts were the product of a private message doing the rounds — basically, a modern version of chain mail — that read: "There are a lot of criticisms among women; instead, we should take care of each other. We are beautiful the way we are. Post a photo in black and white only, write 'challenge accepted' and mention me. Pick nine women to do the same, privately. I chose you because you are beautiful and incredible."
But #challengeaccepted and #womensupportingwomen has taken other forms, too. In Turkey for example the hashtags are being used to draw attention to domestic violence, following the murder of university student Pınar Gültekin by her ex-boyfriend. In that case, the black and white photos mimic the kind that appear in newspaper and television reports about slain women.
For the most part, though, Instagram users will see the more Hollywood take, with phrases like "lift each other up" and "spread the love" dotted through the caption.
The New York Times theorised that the American boom of the challenge may have been sparked by US Congresswoman Alexandria Ocasio-Cortez's empowering speech in the House of Representatives last week, in which she railed against the sexist treatment she — like most women — has experienced in her day-to-day life.
Somehow, the outlet proffered, that groundswell of feminist solidarity manifested into more than 3.8 million posts like this.