Why is the sky blue?

Well, sunlight, which we see as white light, is in reality a mixture of
the colors red, yellow, green, blue, orange, and purple. When sunlight
passes through the earth's atmosphere these colors are deflected and
scattered in many different ways by atmospheric molecules and other
strange things in the air.

The colors purple and blue are deflected the most, which causes these
colors to scatter all over the sky. Because the human eye can detect
blue easier than purple, it is blue you see when you look up at the sky.