Answer:
White light from the sun is composed of many different colors of light. Each color is associated with a different wavelength. Some wavelengths of light are short and some are long. As light travels from the Sun to the Earth, it travels in a straight line unless it bounces off of something. As these wavelengths of light enter our atmosphere on their way to Earth, they may encounter things like dust or drops of water vapor in the air. If a wavelength of light does encounter one of these particles, it will bounce off, or "scatter."
The sky is blue because of something called "Rayleigh scattering," in which longer wavelengths of light are more able to pass through these particles of dust and water vapor without scattering. This means that colors like yellow, orange, and red mostly pass straight through and do not get scattered. Blue light, which has a short wavelength, can actually get absorbed by gas molecules in the air and then scattered in all different directions. This explains why the sky looks blue everywhere - it's being scattered like crazy!
http://www.sciencemadesimple.com/sky_blue.html
