Earth's moon (about 239,000 miles from Earth)
(That's almost ten times as far as going all the way around the Earth)
Nobody knows exactly how the moon formed, but this is one possibility: after the planet Earth formed (along with the other planets), about four and a half billion years ago, during the Hadean Eon, there were still a lot of leftover bits of rock going around the Sun that were not part of any planet. One especially big chunk of rock smashed into the Earth. Most of it stuck and became part of the Earth, but some bits splashed off the Earth. Some of these rocks got caught in Earth's gravity and began to orbit around the Earth. About 45 million years after the Earth formed, many of these rocks stuck together to become the moon. Other planets often have moons too, though they didn't all form in exactly the same way.
At first, the moon was probably much closer to Earth, and spun around much faster than it does now. Earth's gravity, pulling on the moon, has gradually slowed it down so that now the same side of the moon is always facing the Earth. All the moons on the other planets do this too.
The oldest rocks on the moon are about as old as the oldest rocks on Earth, because the Earth and the moon cooled down at about the same time. Like the Earth's rocks, most of the rocks on the moon's surface are made of silica - a combination of silicon and oxygen atoms. This surface is about 60 miles thick on the side away from us, and 100 miles thick on the side towards us. That's because the Earth's gravity has pulled more of the crust to our side of the moon. Inside the moon, it is hot, but not as hot as Earth because the moon is much smaller and cooled off faster. It's not hot enough anymore for tectonic plates to shift around as they do on Earth. The core of the moon, like Earth's core, is probably made mainly of iron.
On the surface of the moon, it's like a rocky desert. The sky looks all black because there is no atmosphere to reflect the sunlight and make it seem blue. That's because the moon is so small, it hasn't got enough gravity to hold on to very many light molecules like oxygen or hydrogen, and they just float off into space.
The moon circles around the Earth just as the Earth circles around the sun, because of centrifugal force and gravity. The moon doesn't have any light of its own - it's just a ball of rock. The moon reflects the light of the sun. Depending on where the moon is and where the sun is, and where you are on the Earth, you may be able to see all of the side of the moon that's lit up by the sun (we call that a full moon). Or you might only be able to see a tiny sliver of the part of the moon that's lit up by the sun (we call that a new moon). As the moon goes around the earth, we cycle between new moons and full moons every month - it takes about a month for the moon to get all the way around the earth, which takes us from one full moon to the next full moon. That's why we call it a month, after the moon. Here's a short video showing how this works (the big ball in the middle is the Sun, the colored ball is the Earth, and the tiny ball going around the Earth is the Moon):
When the earth gets between the moon and the sun, the earth's shadow falls on the moon. The earth's shadow that blocks the sun's light from reaching the moon and creates an eclipse of the moon.