"The Simple Answer..."
Why is the sky blue?
The Simple Answer:
Air molecules preferentially scatter blue light from the sun.
More Detailed Answer:
Light travels in waves. There are perpendicular fields, called oscillating electric and magnetic fields. If the light is in the range of frequency we can see, then it will appear blue. Because of the altitude, the air particles in the sky vibrate at a high frequency we can see, and so our eyes interpret this as blue.