The History of Hidden Odd Ones
Finding the one thing that is different in a crowd is an old puzzle from children's pages and a well-known test of attention.
Puzzle pages
Pages full of small pictures in which one thing is different have long been part of children's magazines and puzzle books. They train careful looking, and they need nothing but a pencil.
A test of attention
In the 20th century psychologists used pages of letters and signs to study attention: you cross out every sign of one kind among many that look alike. Mirrored letters such as b and d are hard for many children who are learning to read, which makes them a good, gentle exercise.
This version
All signs here are simple forms of our own: circles, squares, faces made of a few lines, letters and digits. Every page is made by the program from a fixed number, so each round is the same every time.