No assembly needed.
With both eyes open, look at the white screen, wall, or paper through one of the tubes you made from a full sheet of paper. Notice that the spot of light that you see through the tube appears brighter than the wall of the tube.
Do the same thing using the tube that you made from a narrow strip of paper. Notice that the spot appears darker than the wall of the tube.
When light receptors in your eyes receive light, they send a signal to your brain. A receptor receiving light also sends signals to neighboring receptors, telling them to turn down their own sensitivity to light. When you look at the white wall without a tube, you see a uniform field of brightness because all the receptors are equally inhibited. When you look through the tube that you made from a full sheet of paper, the spot of light is surrounded by the dark ring of the tube. The spot appears brighter because the receptors in the center of your retina are not inhibited by signals from the surrounding dark ring.
In contrast, light shines through the walls of the tube that you made from a strip of paper. When you look through this thinwalled tube, the spot appears darker because light comes through the wall of the tube, causing the receptors at the center of your retina to be inhibited. This is known as lateral inhibition.