Tired eyes
Numerosity adaptation
Start the timer and stare at the + until the dots change. Which circle has more dots now?
Neither: both have the same 30 dots, in the same pattern, though the left one seems to have more.
Why? Even your sense of “how many” gets tired. After lots of dots, a normal number looks like few, and after few dots, it looks like many.
Picture: Drawn by Science Playground, after Gilisa · Wikipedia(opens in a new tab)