Close your eyes. Gently, rub the lids with your hands. You will initially see blackness, but if you continue, you will begin to see shapes emerge from the rippling nothing before you. Abstract and impossible geometric figures will parade from one eye to the next, and you may experience some visions akin to a video feedback loop.

Eventually it passes, to be replaced by a dull glow, and your lids hurt a bit where the eyelashes chafed against them.