In 1932, psychologist Karl Zener devised a set of five cards for testing extrasensory perception. The cards were intended to have very clear, unambiguous shapes on them, to maximise the likelihood that someone with ESP could detect the correct shape in the mind of another. The design of the cards is iconic. Each of the five cards has a figure with something of its number about it: a cicle (a circle has only one line), a cross (two lines joining at the center), three wavey lines, a square, and a pentagram (a five pointed star).
Twenty-five of the cards have a zener card on them, lying on the table, normally towards the top. To run your own ESP experiments have one person draw a card at random and concentrate as hard as they can on the shape. The person who's ESP is in question, then tries to identify the shape by tuning in to the other person's thoughts. On average, someone guessing at random should get one in every five correct, and it would not be too unlikely to randomly get two or three. To get all five correct by guessing is very unlikely, however (a chance of one in more than three thousand).
Zener cards are not seriously used in this way, any more. There are a number of reported biases and tricks which make it possible to score more highly than chance on the test. As such they are now considered a novelty and design icon, rather than a serious research tool.