Why is this the answer?

The answer is shadow because this clever and wonderful riddle gives us four perfect clues about that dark, dancing shape that follows you almost everywhere. Let's start with the first part of the riddle: Only one color, but not one size. If you look at your shadow outside or in a brightly lit room, what color is it? It is always a dark shade, like black or deep gray. That is why the riddle says it has only one color. But if you stick out your hands or turn your body, what happens to your shadow? It stretches, shrinks, and changes its shape and size constantly! It is never the same size for very long, which matches the clue exactly. The second clue is: Stuck at the bottom, yet easily flies. When you are standing or walking, where does your shadow live? It stays right down there on the ground, or on the floor. That is the stuck at the bottom part. However, if you start running or leaping, your shadow moves super fast right along with you. It seems to zip and race across the ground, just like it is easily flying to keep up. It is a quick little friend! Next, the riddle talks about the weather: Present in sun, but not in rain. This clue is one of the biggest hints! A shadow can only be made when a source of light, like the sun, is shining brightly on something. Your body blocks the sunlight, and the shadow appears. When it is raining, the sky is usually full of dark clouds that stop the sun from shining on you, so your shadow disappears! You need the sun to have a shadow, so it is not in rain. The last line of the riddle is: Doing no harm, and feeling no pain. A shadow is not a solid object, like a rock or a toy. It is just a lack of light. Since it is not really a thing, it cannot hurt anyone or anything, so it does no harm. And because it is only a shape, it can never feel scraped knees or a bumped head. It is a silly and fun way to describe something that is completely harmless. All of these fantastic pieces of information come together to tell us the answer is a shadow.