No assembly needed.
(5 minutes or more) Take two round tubes that you made from full sheets of paper. Put the tubes up to your eyes and look through them at the white screen, wall, or sheet of paper. First close one eye, and then open it and close the other. Does the brightness of the spot appear the same for each eye?
Move the tubes to overlap the two spots. Notice that there is a brighter area where the two spots overlap.
Overlap the spots completely. Does the combined spot look brighter than either spot alone? Find out by closing one eye.
When you partly overlap the two spots, your open eye and brain conclude that the sum of the two spots of light should be brighter than one spot alone. If the spots overlap completely, the brain seems to ignore one of them.