For parents who have trouble making the kids go to bed on time, like I do myself:

These past two nights, I lured the kid into bed with "Aurora Borealis" made with three really garish coloured rhinestones and a small flash-light (and with a plate of water & a plate of water and vegetable oil, since I was curious about how the images would change). Put the rhinestones on the floor or into the plate (they float between water and oil, but if you press on them, they sink) and direct light onto them (swishing them around, holding the light closer or farther, nudging them - sometimes, in water/oil, they overlapped) so that there is a reflection on the ceiling. (You can just oil them, it gets a sharper gleam than non-treated 'stones.)

I am going to buy some identical ones and try coating one with colourless nail polish, one with oil, and try adding salt to the water (or glycerine... or chlorophyll solution in ethanol... or benzene...) to see if there will be some change in my "Aurora". Good thing the kid is five and I have some time to brush up on my optics:)