Designed to improve your ability to alter your perception.
For a different take on the exercise check out Rhythm Play.
Take a blank sheet of paper and a soft pencil or stick of charcoal. Sit down and observe a scene that interests you. Forgetting about objects, names, and things, observe only the play of light and shadow over various forms. Block in the shadows, not with lines but with patches of broad strokes. Do not be distracted by local color; do not worry about reproducing “things”. Let the patches of shade create forms. Spend at least ten minutes on this exercise. Remember, the point is not to create a good drawing or to prove your artistic talent (or lack there of); the point is to experience another way of seeing, in which separate objects disappear and only pattern remains.
Exercise courtesy of The Spiral Dance by Starhawk