Mia Simper, from Indiana, was happily chewing the seeds then spitting the shells out of the passenger window.
What she didn't realise, was that the shells were flying straight back in the car and onto her sister's face.
"I was spitting sunflower seeds out the window on the way home but I guess they found their way back in the car," Simper tweeted.
SImper's sister stayed sound asleep, without a clue that a photo of her sunflower seed strewn face was soon going to be liked more than 14,000 times on social media.
In fact, it's even been dubbed the "best thing on Twitter".
When asked about her sister's reaction, Simper later revealed she wasn't very happy.
But thankfully, she soon got over it.
"My sister was excited at first, but the more people brought it up she was like, ‘Yeah, yeah, I know I’m famous'," Simper told BuzzFeed News.
"I always post pictures and videos of my sister on Snapchat and stuff but to watch my post go viral like this was crazy."