The hidden disc, outlined.
Fooled by depth
A stereogram of letters
Look through the screen, as if at something far behind it, until the letters double and settle. What floats out?
A round disc, floating in front of the letters.
Why? Every row repeats every eight characters, but over the disc it repeats one character sooner. When each eye looks at a different repeat, the repeats that are closer together seem nearer to you, and the disc stands out. It helps to start with your nose almost on the screen and slowly move back.
Picture: Drawn by Science Playground · Wikipedia(opens in a new tab)