Why is this the answer?

The answer is shadow because this riddle gives us two perfect clues about how your shadow changes throughout the day. A shadow is that dark shape you can always see when light shines on an object, like you! It is your own dark outline that follows you everywhere you go. Let us look at the first clue in the riddle: "I crawl on the earth." This describes what your shadow does when the sun is high up in the sky, maybe around the middle of the day. When the sun is almost directly over your head, your shadow becomes very short. It stays really close to your feet and looks like it is lying down flat on the ground. When you walk quickly, your shadow seems to move along the earth just like a tiny creature crawling along the floor. It is flat and low, only stretching a little bit around your shoes. It is easy to step right over a short shadow that is "crawling" like that. Now let us look at the second part of the riddle: "And rise on a pillar." This is a clever way to describe what happens to your shadow when the sun is low. Think about what the sky looks like when you wake up in the morning and the sun is just coming up, or in the evening when the sun is going down. When the light hits you from the side, your shadow gets super long and very tall! It stretches out incredibly far across the ground, and it can look as tall as a huge pole or a "pillar." Your shadow looks like a giant standing beside you, rising high up into the air. It is amazing how your shadow can be both flat on the earth and a towering pillar just by waiting for the sun to move across the sky. The shadow is constantly changing its size and shape, and that is why it is the correct answer to this smart riddle.