Cut any extra flaps from the box, including those that fold in to the sides and front. Tape a blank CD to the inside of one short end, flush against the back wall. Make sure your box can close!
Trace around the end of a paper towel tube twice, in overlapping circles, so you have an opening that’s about 1 and 1/2 times as wide as the tube. Insert the tube at an angle, looking towards the CD.
Also cut a slit on the short edge directly opposite the CD, to let in light. Tape up any other edges where light might slip in.
Now take a peak inside! We found this worked best when we shined a flashlight directly into our slit. One person can shine the light while the other person makes sure the tube is aimed properly at the CD; you’ll see the spectrum of the rainbow appear.
You can also try this with other light sources, including sunlight, and see which one works best!