points together
Hold a pencil by the point end in one hand. Hold another
pencil by the point end in the other hand. Close one eye.
Reach both arms straight out in front of you. The pencils'
erasers should be aimed toward each other and about a foot
apart. Try to bring the two pencil erasers together. You
will probably miss by an inch or more. Try again with both
eyes open. This time it is much easier.
What one eye sees is slightly different from what the other
eye sees. The brain uses the difference in the two views to
judge distance.