Actually, it’s for the same reason as you have misted up windows in toilet cubicles. I don’t know if you’ve ever noticed but toilet windows hardly ever have normal see-through glass in them, they always have sort of fogged up glass, so people can’t look in from the outside and see things that might shock them. Well, whoever made the sky obviously didn’t want us looking straight through it into whatever is beyond – what we might perhaps call the Divine Bathroom – so they made it blue. Otherwise, you’d just be able to see straight through all that air – which is completely CLEAR, obviously – to, well, the universe. Which is NOT your business. THAT’s why the sky is blue.
Anybody got any other ideas? After all, this IS the most asked question on Google today….