Great question! This is one of my favorite topics. The moon is actually “closer to home” than most people think, in fact a lot of the moon is made up of material that used to be on earth! The earth formed about 4.56 billion years ago (that’s a really long time). In the first several hundred million years of earth’s life it was getting hit really hard by asteroids, comets, and meteorites. Impacts of “planetesimals” (small planets) happened very often. At 4.5 billion years ago, a planetesimal the size of Mars crashed into the earth. It hit the earth with so much energy that part of the earth was actually broken off and sent into space. The part that was ripped off was caught by the gravity of the earth that was left behind, so it started to orbit it. That is why the moon orbits the earth. So, the moon is made up of stuff that was once part of this planet. In fact, the rocks that we find on the moon are very similar to rocks that we find in certain places on earth!

I found great information for you on the Web. Please read it and try to watch the second video on the web page; it shows a simulation of the formation of the Moon.

The moon was formed ~4.5 billion years ago, about 30–50 million years after the origin of the Solar System, out of debris thrown into orbit by a massive collision between a smaller proto-Earth and another planetoid, about the size of Mars. Initially the Moon spun much faster, but because it is not perfectly spherical and bulges out slightly at its equator, the orbit slowed down and eventually became tidally locked — keeping the same face toward the Earth. Bulges along the Earth-Moon line caused a torque, slowing the Moon spin, much the same way a figure skater gradually opens to decelerate a spin. When the Moon’s spin slowed enough to match its orbital rate, the bulge was in line with Earth, which is why we always see the same side of the Moon. In our solar system, almost all moons spin at the same rate as they orbit.

The Earth would be a very different place if the moon did not exist. Not only did the Earth slow down the Moon’s rotation, but the Moon is slowing down the rotation rate of the Earth. Since the moon’s formation, the Earth has been slowing its rotation due to the friction of the tides caused by the moon, and in reaction to this exchange of energy, the moon has been moving farther away from the Earth. In fact, at the time of the moon’s formation the Earth rotated much faster than it does today; a day on early Earth was only a few hours long. But the Moon, being small in relation to Earth, will take more than twice the age of the solar system to slow Earth’s spin rate to the Moon’s orbital rate.

The most accepted theory is that while the Earth and Solar System were forming, a large object (about the size of Mars) crashed into the Earth at a slow speed, and small angle (it barely hit us!), knocking some of Earth's mass off. This mass did not go far enough to leave Earth's gravitational pull, but far enough to leave the atmosphere, creating the Moon we know today. It is also hypothesized that this crash is responsible for our tilt leading to seasons as we travel around the sun.

About 4.5 billion years ago, when the Earth was young, an object about the size of Mars collided with Earth. This collision blasted off a large amount of material from the outer part of the Earth and into orbit around the Earth. This material then coalesced to form the Moon.

Good question - until recently, it was thought that the Earth was hit by a Mars-sized object about 4.5 billion years ago, and that the debris blown off from the impact is what formed the moon. I think that theory has come under fire, though, and I don't know what the current preferred theory is.