Let's talk Instagram. Odds are you've got it pulled up on your phone right now. On average, people spend just over 21 minutes every day scrolling through their college friends' brunches and favorite celebrities' selfies. And for us beauty gurus and pop culture junkies, Instagram is also where we get a ton of hair and makeup inspiration.

And it goes both ways: Celebs, makeup artists and other beauty icons love sharing their favorite products and looks on Insta'. We owe some of the biggest trends in beauty to Instagram: Kim K-style contouring, strobing, fleeked-out eyebrows and galaxy hair -- all made popular on the app.

So, since Instagram has played such a big role in beauty lovers' lives in 2015, we decided to put together a little Instagram yearbook. Take a backwards scroll (see what we did there?) through the most memorable celebrity hair transformations, makeup looks and brand launches from the past year. (Think: #ThighBrows, sheet mask selfies and Kylie Jenner's lip injections.) And before a whole new year begins, discover our favorite Insta-beauty moment of 2015.

Sheet masks were all the rage in 2015, as evidenced by dozens of new launches, from Yes To and Dr Jart+ to Dolce & Gabbana.

Celebrity sheet mask selfies became a veritable thing on Insta this year (we know, what a world), but none slayed quite like the one posted by Lady Gaga. Who else rocks a sheet mask in giant shades, nail art and diamonds? Only Gaga.

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Having seen nothing but the signature half-up ponytail from Ariana Grande for the past several years, we started to believe she woke up that way.

So it was quite the shock (and delight) to see this rare Instagram from the tiny songstress, featuring an insanely stunning collection of natural, healthy curls. We hope to see more of them in 2016 -- but since she seems fairly attached to the flat iron, at least we have this little window into the real Ari from September 2015. (We even got to see the right side of her face!)

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We always thought stretch marks were something us nameless commoners experienced and never perfect, thin supermodels. That is, until Chrissy Teigen posted this Instagram back in April 2015 of what have arguably become the most famous stretch marks on the Internet.

"Stretches say hi!" captioned the "Sports Illustrated" model of her comfortingly normal-looking thighs. For its stripped-down realness, this beauty Insta lands in the No. 9 spot on our list. (Though if we were ranking nicknames for stretch marks, we're pretty sure it'd be No. 1.)

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Kardashians and makeovers are two of the Internet's favorite things, so it's no surprise that the Insta-universe freaked when Kim K revealed she'd gone platinum blonde overnight back in March for Paris Fashion Week.

Kim didn't keep the look for long, but we were obsessed with it while it lasted. Colorist Lorri Goddard nailed the ashy platinum color (tricky to achieve, coming from ultra-dark brunette), making this one of the most memorable hair color makeovers of 2015.

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