The Collisional Ejection Theory

This theory holds that the Moon formed from debris ejected from the Earth when a large object (possibly as large as Mars) crashed into the Earth.

-Impact - A large planetesimal hits Earth, ejecting material which forms the Moon.

-The moon has exactly the same oxygen isotope composition as the Earth.

-the stable isotope ratios of lunar and terrestrial rock are identical, implying a common origin.