Our new installment of the “Illusions” column in Scientific American Mind, out yesterday, is dedicated to “Folk Illusions,” the misperception tricks that school age children play on each other at recess and sleepover parties. The tradition goes back to (at least) several centuries. K. Brandon Barker of Indiana University and Claiborne Rice of the University of Louisiana at Lafayette have categorized many of these childhood games, and want to hear more, especially from readers who grew up in non-Western cultures. Please share the folk illusions from your childhood in the Comments section of this post.

The current “Illusions” article in Scientific American Mind features three such folk illusions. Here’s a fourth one, observed in the field by Baker:  

Ping Pong illusion
In June of 2012, Barker visited the Lost Bayou Scout Camp in Louisiana to conduct fieldwork observations of the children’s activities (their ages ranged from 10 to 17). While walking from one camp area to another, he observed a game he had never seen previously, or even heard of. Two boys, ages 12 and 15, seemed to be playing Ping Pong without a net or table, about 25-30 feet away from Barker. Half a dozen other kids were watching the game and cheering. The players were using some kind of hand-held paddles to hit the ball, and doing so at almost superhuman speeds. Barker could not believe his eyes… and then realized that his eyes were not to be believed indeed. The children were playing Ping Pong without a ball. As Barker got closer to the game, he saw that the ‘paddles’ were Styrofoam cups. Each boy was holding the cup’s open end against the palm of his dominant hand, so that his middle finger would flick the edge of the cup from the inside, at the time it hit the imaginary ball. The resulting sound, aided by the additional cues of the illusory game—the jumping and running of the players, the consistent head movements and cheering of the crowd—evoked the thwack of a real ball hitting a paddle. Barker later learned that Lost Bayou camp boy scouts had been playing variants of the same game since at least 2005. The illusion likely works due to similar mechanisms as the magic ‘vanishing’ of flying coins and other objects performed by conjurers, where no object is actually thrown in the first place. The kinematics of the simulated Ping Pong game are so close to the real thing that they overpower the visual system’s motion detection circuits, making them fire as in the presence of an actual moving object. The synchrony between the finger thump and the paddle action of hitting a ball serves to enhance the implicit motion visual cues in the performance.