Thread the Needle

Stand the washer on its edge, using the lump of clay to keep it upright.

Hold the lump of clay with the washer in one hand and the pencil in the other. Orient the washer so the edge—not the hole—is facing you and extend your arm in front of you.

Close one eye and try to put the pencil through the hole in the washer.

Open both eyes and try again.

One of the clues your brain uses to judge distance and depth is the very slight difference between what your left eye sees and what your right eye sees. Your brain combines these two views to make a three-dimensional picture of the world.

Try this experiment again with one eye closed. But this time, move your head from side to side as you “thread the needle.” People who have lost an eye can learn to perceive depth by comparing the different views they obtain from one eye at two separate times.

Stretch a string from just under your nose to the end of your outstretched arm. You will see two strings stretching out in front of you. Look at the string with just your left eye, and then with just your right eye. Notice that the two strings are separate images—one from each eye. The two strings cross at the point on which your eyes are focused. Try looking at different points on the string and notice how the crossover point moves.