Why is the sky blue?

On a clear cloudless day the sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light.  When we look towards the sun at sunset, we see red and orange colors because the blue light has been scattered out and away from the line of sight. If you want a more detailed explanation look here.

Hi, 

Just wanted to elaborate a bit. In the atmosphere we can find molecules of gas, vapor, dust and other substances. When light hits these molecules it can be deflected in different directions. When light hits the gas molecules, a phenomenon we see is light being absorbed by the gas molecule. After a while this light can be emitted by the gas. Not all light waves are absorbed but only certain wavelengths. With these fact we can analyze why the sky is blue.

Among the different wavelengths blue light is the one absorbed by the gas molecules. After a while this light is emitted by the same molecules in all directions. Some of that light reaches our eyes and hence we see a blue sky.