At first glance Katy Perry's dress looked simple and glamorous...
But when the lights went out the star lit up the room!
Katy's dress had rows of fairy lights inside it which made it glow.
Katy outshone all of the other celebs at the Metropolitan Museum Gala Benefit in New York.
The lights were powered by a tiny battery inside her skirt.
Harry Potter star Emma Watson was also at the gala, wearing a very similar dress...
But Katy's dress was definitely the brightest of the bunch!