I was just joking around InspectorD.
But, I did think that people would be entertained by knowing the reason why blue jeans are darker when they're wet or why a wet t-shirt is more transparent than a dry t-shirt. This is the kind of thing you can have fun with. Next time you walk into a bar and notice someone you'd like to talk to, ask them why the head on their beer is white even though nothing inside that foam is white in colour. (It's exactly the same reason why clouds, snowbanks and waterfalls are white in colour. Post again if you still can't figure it out.)
Anyhow, a good way to entertain kids for a day or two is by making test tubes disappear. I turns out that Pyrex glass (from which test tubes are made) and ordinary corn oil from the grocery store have refractive indices that are nearly identical. Pyrex glass has a refractive index of 1.470 whereas corn oil's refractive index varies from 1.455 to 1.467. (you may have to try different brands of oils for best results) That's close enough to 1.470 to make for very little reflection and refraction at the glass/oil interfaces. That is, light passing through the submerged test tube will behave very much like it would if the test tube weren't even there. And, what does that mean in practical terms? It means that the test tube will seem to disappear from sight as you immerse it in the corn oil.
Your kids will spend hours running around the neighborhood showing this "magic" trick to their friends. It's a novelty to see something you don't expect to see (or, in this case, not see something you expect to).