With the cover on the box and the hinged window closed, close one eye. With your other eye, look through the viewing hole at the construction paper circles. Lift the end of the box cover closest to you and slide it away from you until adequate viewing light reaches the inside of the box. Push or pull on the straws from the outside rear of the box until the mounted circles appear to be the same size. Note that your depth perception is almost nonexistent: both circles look essentially the same distance away, and it’s very difficult to judge which circle is closest to you.
Fold down the viewing window and look with both eyes to see the actual positions of the circles. (You may have to move your head back a little from the box to get both circles focused.) Note that depth perception is now a factor and the circles no longer look the same size or the same distance away.
Position the small disk a couple of inches closer to you than the large disk. With both eyes open, look through the window at the disks. Notice that you have no trouble establishing their size and distance.
Now close one eye and notice that it may be much more difficult to tell whether the small disk is now actually a small disk that’s close to you, or a large disk that’s far away. You can also use the dime and quarter by themselves, without building the box, to illustrate the same principle. Close one eye. Holding one coin in each hand, move them toward or away from your viewing eye until they appear to be the same size. A solid-colored background gives less distraction than an irregular background. A very bright, solid-colored background works best, so that the coins appear essentially in silhouette, and their features cannot easily be distinguished.