This is actually a very deep and intelligent question. This website is dedicated to answering that question in the clearest and most thorough manner possible. We are not going to only give you the answer, but hopefully we will make you think as well. So let's figure out Why is the sky blue?
This website is presented at the high school or college level. If you want a one page quick version, or a good version for kids, see Why is the Sky Blue - For Kids.
My first question to you is: Why is the sky not black? Let's do a quick thinking experiment. Imagine you are sitting out in space on a green marble, and someone shines a flash light at you. Or suppose you are in a very large blacked out room and someone far away turns on a flashlight. This is illustrated in Figure 1: