“Daddy, why is there a moon?”
Ah, an old story. In the beginning, Mother Earth stood among a throng of smaller worlds. She was special, though, favored by grandfather Sun. He shown his pride on her all the time. The other worlds swirled around her and taunted her, jealous of her special place. They even hit her, sometimes. She was strong, though, and those blows only made her stronger and bigger. One day, another large world became jealous of the place Mother Earth held in the solar system. She was almost as large as Earth, why shouldn’t she get the favored spot? So she thought to herself. She swung close by once, shooting past the Sun, looking for any reaction. Sun didn’t notice, he was still watching Earth. As she came around for another attack, earth braced herself. Her face reddened, and she heated up. The two clashed, and a great spray of lava spewed from the wounds they inflicted on each other. Mother earth staggered under the blow and almost broke apart. She was just strong enough to hold her shape, though. The other world, also close to breaking apart, turned. Blinded by her anger, she attacked Mother Earth again. This blow was nearly as great as the first, and both worlds lay broken. A small part of each remained, though. Enough to gather back together. As they both brought their pieces back together, they realized that they had become one. The Earth and this new world had joined, both were now made up of the same rock. There was no way to tell which came from which. Realizing that they could no longer fight, Mother Earth named her new companion the Moon. They have shared the warmth of Grandfather Sun ever since. Now, their fights are good natured, as one or the other will jockey for a momentary place in front of the other.
The Moon was created when another body in the primordial Solar System collided with Earth. Space.com link