I came across this article on the BBC website yesterday. It talked about how parents feel that they are being routinely baffled by science questions their children asked. Apparently, the top three questions that parents don't like to hear are: "Where do babies come from?", "What makes a rainbow?" and "Why is the sky blue?" (The article refers to the website Science: [So what? - so everything] which is great reading.)
To begin with, two things spring to my mind here. First, are parents any more baffled now than they used to be? What with ready access to answers through the internet, perhaps it is not surprising that some adults think their children know more than they do. The question "Why is the sky blue?" is not an easy one - I'll come back to that - I am not surprised in the least that parents struggle with it. And secondly, two out of the top three are physics questions. Is every child a budding physicist who just needs a bit of nurturing along that line? Let's hope so.
So why is the sky blue? Without looking at the answer on the website, I would say it's because of what light from the sun does when it hits tiny dust particles in the atmosphere (the sky). The light from the sun is made up of lots of colours (see the rainbow question) and the dust particle is able to change the direction of the blue light much more easily that it can change the direction of the red light. (We say the blue light is scattered much more strongly.) So the blue light appears to go in all directions, and the red light doesn't, so the sky generally looks blue. (This is also why sunsets are red - particularly after volcanic eruptions - the blue light from the sun gets scattered out of the way so when you look at the sun you only see the red light). And i would leave the answer at that for a child, but of course there will always be those that say "But why is the blue light scattered more strongly?"
That is rather harder to answer, and I'm going to cop out at this stage. It was my third year at university when we actually went through the reason for this, and to put it into words a child can understand is going to take a bit of thought. Any suggestions from readers?
Incidently, Science: [So what? - so everything] gives child-friendly answers for these three and lots of other questions, including the one about where babies come from. Great idea.