Fooled by depth
Autostereogram
Hold the picture close, look “through” it as if at something far behind, and slowly move back. What appears?
A 3D shark.
Why? The pattern repeats with small shifts. When each eye looks at a different copy, your brain reads the shifts as depth, just as it does with your two eyes every day.
Picture: Fred Hsu, CC BY-SA 3.0 · Wikipedia(opens in a new tab)