Watch out Hannah Montana! There's a new show starring Miley Cyrus that's blowing up online.

The Miley and Mandy Show – starring 15-year-old Cyrus and her best friend Mandy Jiroux, a dancer from the Best of Both Worlds tour – debuted on YouTube last month and the first four episodes have combined for more than 1 million hits.

The impetus behind the new Web hit? They were just teens being teens.

"Miley got a new computer and [Miley and Mandy] just decided to make videos together," says a source close to Cyrus. "They do it all themselves. They pick the music and Miley edits it together. It's just two girls having fun."

While most of the videos take place in Cyrus's bedroom in her family's Toluca Lake, Calif. home, the singer filmed the recent Miley and Mandyless episode featuring Akon's song "Lonely" backstage at The Tonight Show.

"She was sitting around after sound check and had four hours until the show. She was really bored," says the source. "She was talking to Mandy on the phone and came up with the idea of doing the video about being bored and apart."

Cyrus crafted the video so that the segments in which the two best friends were separated appeared in dreary black and white and scenes of the twosome together popped up in color.

Any plans for more? "It's really organic," the source says. "Whenever they have time, they just goof off."