Why is this the answer?

The answer is moon because this is a truly marvelous riddle that describes our glowing, nighttime neighbor with wonderful clues. Let's look at each part of the riddle to see how it perfectly fits the moon, using our best detective skills. The first line is "Always old, sometimes new." Think about how long the moon has been floating up in the sky. It has been there for an incredibly long time, billions of years even, so it is definitely "always old." But we also know that the moon changes the way it looks to us from Earth. When we see a tiny crescent shape, we often call that a new moon. Then, when it grows into a big, bright, perfect circle, we call it a full moon. Because its appearance keeps changing, it is always appearing "sometimes new" to us, even though it is very old. Next, the riddle says, "Never sad, sometimes blue." Well, the moon is not a person, so it can never have feelings or be sad. That part is easy! The phrase "sometimes blue" is a clever way to trick you. We sometimes say things like "once in a blue moon" when something happens very rarely. Scientists also know that under special conditions, like when there's smoke or dust in the air, the moon can actually appear to have a slightly blue color! It is a really fun and tricky clue for a third grader to figure out. The third line is "Never empty, sometimes full." The moon is never truly empty because it is a giant, solid rock floating in space. It is always there, even on nights when the clouds hide it or when the sky is too bright to see it. But when we look up, we see the moon go through its phases. Sometimes only a small part is lit up, and other times it is a big, round, "full" moon shining down on us. This part of the riddle perfectly describes the changing shapes we observe in the night sky. Finally, the riddle says, "Never pushes, always pulls." This is the best clue because it talks about science! The moon has an invisible, powerful force called gravity. The moon’s gravity is so strong that it reaches out across space and pulls on all the oceans and rivers right here on Earth. This pull is what causes the high tides and low tides we see at the beach every single day. The moon is constantly pulling on our planet, never pushing it away. All these clever descriptions together tell us the secret answer is the moon.