A blue-and-black dress has got millions of people all over the world seriously confused.
When looking at a photo of it, some people see it as gold and white, while others see it as blue and black.
Colours can appear different depending on what kind of light they're in. Something that's blue, for example, can look totally different in sunlight, than under streetlights at night-time.
But even when it's the same photograph, some people see blue, and others see white.
It's caused a big fuss online.
Taylor Swift said it got her confused and scared. Demi Lovato said that her mind was blown.
Watch Hayley's report to find out more...