This is Meeiah Tolentino, a 16-year-old from Alaska, and her little sister Rain, 3.
Like many little sisters, Rain loves following her big sister around and has a lot of curiosity about the world, Meeiah told BuzzFeed News.
"She loves following everyone in my family around," she said. "I mean, she's 3 years old — little kids are curious."
A few days ago, Meeiah had her period and went to the bathroom. She said she was in a rush and didn't bother to check if the door was fully closed.
Rain innocently followed her sister into the bathroom and got a shocking lesson in womanhood. She totally freaked out, and thought her sister was dying.
Meeiah said Rain was only in there with her for a few seconds before her cousin pulled her out, but she immediately started crying.
"I went in my room and saw her bawling," she said. "She was yelling, 'You need to go to the doctor,' and 'You're going to die because you're bleeding!'"
Meeiah captured her reaction on video to show her family, but then gave her sister a hug and assured her she was OK.
"I even let her pretend to be a doctor and fix me up," she said. "Two minutes after the video she totally forgot what of happened and went [back] to her normal, happy, self."
Meeiah shared the video on Twitter "for a few laughs," and it has since been retweeted more than 70,000 times. Many people said they or their siblings had similar introductions to womanhood.
Meeiah said it's been a little weird seeing her sister all over social media, but most of the reactions have been positive.
"Some of them are actually agreeing to my sister's reaction when they're on their period," she said.
She added that her family has also been shocked by the reaction to the video.
"My parents are very surprised that she's all over the place ... But they thought it was funny, and now they're treating my little sister like she's a superstar," she said. "It's just surprising to everyone that the video went viral."