Two pictures in one
Pyramids or tunnels?
Do you see pyramids sticking out, or tunnels going in?
Both are right. Keep looking and each square will switch between a pyramid seen from above and a tunnel seen from inside.
Why? The squares get closer together on one side, the way the sides of a pyramid or a tunnel would look. Nothing tells your brain which way it goes, so it picks one, and then the other.
Picture: Drawn by Science Playground · Wikipedia(opens in a new tab)