She's Just 12 Years Old But... Watch What Simon Does After She Opens Her Mouth

It sounds like you're describing a viral video of a 12-year-old girl who impresses Simon Cowell with her singing on a talent show.

Again, there have been several instances of this type of video circulating on social media over the years, so it's difficult to say which specific video you're referring to without more information. However, Simon Cowell is a well-known talent show judge, having been involved with shows such as "Britain's Got Talent" and "The X Factor," among others. Cowell is known for his blunt critiques and high standards, so when he is impressed by a performer, it can be a powerful endorsement of their talent.