(CNN News) - It was supposed to be a lesson in social media and online use.
A teacher posted a picture on the Internet to show her student's how quickly what you put online can spread.
Little did she know her lesson would go viral.
For teachers who think few students pay attention in class, meet Julie Culp.
“I decided that I was going to post a photo on Facebook,” Culp said.
The photo is one of Culp, the counselor at Indian Lake Elementary School in Hendersonville, Tenn., holding a sheet of paper explaining she's teaching students how quickly a photo can be seen by lots of people. It said "please click like,” and people did.
The photo quickly captured the eyes of hundreds. Then, it exploded.
As days progressed, three and a half million people liked it.
“Once it got off of my page, I really can't keep up. I know that there was one Facebook page that posted and I think they got around a million likes,” Culp said.
It hit Twitter, Google +, Reddit and several other social media sites.
“You've gotta think before you do it, you can't just post something up there and not think that it's a big deal because it can be a big deal,” she said.
And her lesson is being heard loud and clear by her intended audience, her students.
“Who knows, like if you get jobs, they may check your Facebook, Twitter accounts, anything you have,” one of her students said.
Culp's response: “If you don't want everyone to see it, you don't need to put it out there.”