Chances are, as a kiddywink, you looked up to your Mom or Dad and asked "why is the sky blue?" Just in case you've forgotten, or your parents told you a whopping fib, io9 has come to the rescue.
For the full explanation, you'd be best off heading over to our sister-site io9 now, but if a summary is what you're after, stay put right here.
The brief explanation is to do with the sun—specifically, the "Rayleigh Scattering" of the light. It also explains why sunsets are red, with the term coined by English physicist Lord Rayleigh in 1871. When the sunlight hits smaller particles of oxygen and nitrogen in the air, the light then "scatters" in various directions. The blue light, causing a lovely blue sky, comes from light wavelengths smaller than 400 nanometers, and red light for sunsets occurs when the wavelengths are around 700 nanometers in size.
Looking above you, the whole sky appears blue because so much scattering is being done. It's not constrained to just the area around the sun, because so much scattering is happening that the whole sky is filled with the blue light. This happens until the early evening, when the sun dips and there's less light in the air. [HowStuffWorks and Science Encyclopedia via io9]
Image Credit: Yves