2This book provides a welcome departure from the more traditional coloring books available today. But beware, the designs herein may boggle your mind, and confound your eyes; they are all visual illusions. Does the cube go in or out . . . is the line straight or curved? You'll have fun deciding which while coloring them at the same time. Almost any medium can be used, crayon, watercolors, felt tip pens, tempera, for results that may be more confusing than the original.