This demonstration gives the qualitative feel of the randomization task used to study categorization of multi-dimensional stimuli. The three rectangles arranged in an horizontal row correspond to left, center, and right keys in an experimental chamber. Clicking your mouse on an illuminated key moves the procedure to the next stage. (A key is illuminated when it is blue, green, or red.) Begin by clicking on the center illuminated key. This response produces a yellow rectangle between the left and right keys. Clicking on the correct side key leads either to reinforcement (a yellow light beneath the three keys, corresponding to the delivery of food in a hopper to a pigeon), or to a blackout and then a new trial. Most rectangles are more strongly associated with reinforcement for one side key than for the other. However, any rectangle can on different occasions be an exemplar of either category. The categories are in that sense "ill defined." See if you can discover what the two ill-defined categories are. (If you have trouble, see the corresponding text.)