Why is this the answer?

The answer is shadow because this clever riddle uses amazing word pictures to describe how your shadow acts from the moment the sun comes up until the middle of the day. It is a puzzle that describes the simple, fun science of light. The first line gives us a great clue: "Each morning I appear to lie at your feet." Imagine stepping outside when the sun is still low, just peeking over the horizon. When the light shines from a low angle, it creates a very long shadow that stretches far away from you on the ground. Because the shadow is so long, it seems to be stretched out flat, almost like it is resting or "lying" near your feet, waiting for you to start your day. It is a long, dark shape that you can easily spot in the early hours. The next line talks about how the shadow moves: "all day I follow no matter how fast you run." Your shadow is a loyal companion, but only when it is sunny outside. If you decide to race a friend across the playground or just walk slowly from your desk to the pencil sharpener, your shadow is always there. It copies every move you make. If you jump, it jumps. If you stop, it stops. You can try to run away from it, but you will never be fast enough because it is created by the speed of light and follows the rules of physics. It proves that you always have a friend to play with when the sun is shining. Finally, the riddle describes what happens around lunchtime: "Yet I nearly perish in the midday sun." Midday is the time of noon, when the sun is at its very highest point in the sky, right over your head. When the sun is shining straight down from above, your shadow shrinks until it is very, very small. It almost disappears right underneath you, making it hard to see. The riddle says it "nearly perishes" because it is so tiny and close that it practically vanishes for a short time, only to grow long again in the afternoon as the sun starts to set. This riddle is a fun way to teach you about how the sun's position changes the size and location of your faithful shadow.