She sang a Shepherd's song from the medieval days - 33 seconds later a magical thing happend!
In the past, it was normal to use shepherd's calls to gather herds of cows together.
But how do cows respond to calls of shepherds today? Jonna Jinton of Sweden decided to try.
"It was midsummer night, and I decided to go out and call the cows", she wrote on YouTube. "I thought there might be something deep within them that makes them remember the voices and sounds of ancient times, in times when people sang them from a distance".
The film, already watched a millions of times, is simply magical. See how the cows are responding to the magical voice of Jonna - it is just beautiful!