Explaining to a six-year-old

Albert Einstein is said to have said that "If you can't explain something to a six-year-old, you really don't understand it yourself." Employing a basic rule of logic, it can be restated as "If you yourself understand something, then you can explain it to a six-year-old."

This is a very optimistic view, and my own experience supports it. Of course, I would explain things differently to a 6-year-old, a 16-year-old and a 26-year-old with a relevant high academic degree. Still, it can be explained to a child in a way that will be truthful and will give the child the great feeling of comprehension. It will strengthen the child's assumption that they can understand anything - nothing is too difficult. Wouldn't you want your child to have that self-assuredness?

Why is the sky blue?

A 16-year-old with fair education can probably use a simple scientific explanation, such as http://www.sciencemadesimple.com/sky_blue.html

A 26-year-old with a master in theoretical physics can be interested in a quantum-mechanics explanation of exactly why a certain molecule may absorb photons of certain wavelengths, why they may release them, and how to deal with the sad fact (or claim) that the atmosphere doesn't seem to contain the right type of molecules.

A 6-year-old can relate to a picture, such as the one from http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html , with an explanation along these lines: The light from the sun has all colors in it; some colors, like the red go straight through the air; other colors, such as blue, collide with the air and bounce back towards our eyes, like from a mirror, even if we were looking at the sky and not at the sun.

Different explanations can be created for different age groups, different sociological background, different personal tendency of child and tutor, etc. Some explanations may involve experiments or observations.

For a while now I have been contemplating a wiki-style project, with Frequently Asked Questions answered in many different ways. Mere contemplation doesn't make it happen, but with a practically infinite Internet, someone is likely to do it one of these days.