Fooled by depth
Wiggle stereoscopy
Does the picture look flat, or deep?
Deep, even with one eye closed, though it is only two flat pictures taking turns.
Why? The two pictures are taken a little apart, like your two eyes. Flicking between them, near things jump more than far ones, and your brain reads the jumps as depth.
Picture: Drawn by Science Playground · Wikipedia(opens in a new tab)