Why is the sky blue?

My 3-year old preschooler recently attended a summer kindergarten workshop. In one of their activities, they were assigned to color various objects such as sun, trees, moon, sky and so on. Suddenly, she asked me, "mommy, what is the color of the sky?". I immediately replied without thinking, "it's blue baby." Have you ever thought why the sky is blue? This video might help us know why.