How can looking at an outstretched thumb with one eye at a time demonstrate parallax?

Looking at an outstretched thumb with one eye at a time demonstrates the principle of binocular parallax (or binocular disparity), which arises because your eyes are separated by a distance in your skull and therefore receive slightly different images of the same object.12

To perform this demonstration:
* Hold your thumb at arm's length and fix your gaze on a distant object behind it.
* Close your left eye and note the position of your thumb against the background.
* Switch to closing your right eye while opening the left, keeping your head and thumb perfectly still.

You will observe that your thumb appears to "jump" or shift its position relative to the background. This occurs because each eye views the thumb from a different angle, creating a difference in the direction of the object as seen from two distinct points. The brain uses this disparity between the two retinal images to perceive depth and distance; the greater the shift (parallax), the closer the object is perceived to be.132