Why these two photos, taken two years apart, are being shared around the world.

A US student’s composite photo uploaded to Twitter is sending the platform into a frenzy, starting its own viral trend.

On Monday, Caitlin Crowley, a 17-year-old Florida student, uploaded two photos side-by-side. One was in freshman year, and the other in senior year.

On one side, she stood awkwardly beside her male prom date, and on the other, she passionately embraced her girlfriend.

“Freshman to senior year, does this count as a glo up?” she captioned the image.

A ‘glow-up’, something I absolutely did not have to look up and confidently knew at the forefront of my brain (ahem), is an incredible transformation.

Or, in Urban Dictionary‘s words – AKA the purveyor of all things good and true – it means, “to go from the bottom to the top to the point of disbelief”.

In an interview with Refinery29, Crowley said she met her girfriend through mutual friends, though they do not go to school with one another.

“[We] quickly became best friends, and then about a year later we realised we were actually romantically in love with each other and began dating summer of 2016,” she says.

In a separate interview with Buzzfeed, Crowley admitted she was overwhelmed with the sheer volume of responses the photo received.

“Hearing the stories of complete strangers and their own coming out fears really hit me hard, because when I was first coming to terms with my sexuality I didn’t know a single lesbian,” she said.

“To anyone who’s still in the closet: take your time, you are not alone, and you are valid!”

Since she posted, Crowley’s tweet has been liked more than 133,000 times and re-tweeted more than 30,000 times.

Other teenagers are also posting their own versions of glow-ups to the platform, quickly turning the innocent photo into a swift and far-reaching movement.