The challenge is essentially a topical variation on the "Dolly Parton Challenge" that went viral earlier this year.
But this time, celebrities aren't sharing funny photos of themselves that are appropriate for LinkedIn, Facebook, Instagram and Tinder. Instead, they're posting a grid of facial expressions showing how their mood has changed each month from January – before coronavirus really took hold in many countries – right up until September.
Witherspoon's original meme begins with her looking polished and positive in Legally Blonde and ends with her looking rather more frazzled in the emotional hitchhiking drama Wild. It's an instant classic post.
Witherspoon's original post has clearly captured a mood because fellow stars including Viola Davis, Mindy Kaling and Kylie Minogue have all shared their own takes on the 2020 Challenge.
The cult Instagram account @loveofhuns has got in on the act too, creating an oh-so-British "2020 Challenge" featuring Kat Slater from EastEnders. It might just be the most relatable one of all.