The geometric patterns and checkerboards you see when pressing gently on your closed eyes as a kid (phosphenes). As a child, I used to treat it like a built-in

The geometric patterns and checkerboards you see when pressing gently on your closed eyes as a kid (phosphenes). As a child, I used to treat it like a built-in screensaver or kaleidoscope before sleep, completely unaware that it was just mechanical stimulation of the retinal cells producing false electrical signals.