We take the sky for granted. It is just there. Yet, whether it is covered with cloud, or clear, we do not consider the fact that, during the daytime, all of the sky is somewhat light. In particular, on a clear day, the sky appears blue. The blueness itself is remarkable, given that the sun is yellow, but why is the sky any color at all?

. Image: PiccoloNamek at En.Wikipedia, Creative Commons Attribution-Share Alike 3.0 Unported.