| Visitors are invited to 
              demonstrate the usefulness of binocular depth perception by trying 
              to thread a needle with only one eye. They can "thread" 
              the giant needle in the exhibit with a long metal rod held about 
              at arm's length. Most people don't even come close when they first 
              try it with one eye. After several attempts, however, they begin 
              to get closer and closer to the target. People who have no stereoscopic 
              vision learn to use other clues in place of two eyes. |   |