Lemons
The University of Northern Iowa once offered a general art course that included a most unusual exercise. The teacher brought to class a shopping bag filled with lemons and gave a lemon to each class member. The assignment was for the student to keep his lemon with him day and night—smelling, handling, examining it.
Next class period, without warning, students were told to put their lemons back in the bag. Then each was asked to find his lemon. Surprisingly, most did so without difficulty.