[Question_Assumptions]

The current theory, because simulations have show that it works, is a Mars-sized planet hitting "Earth I" at an oblique angle, destroying both planets in the process but leaving enough debris in orbit to form the moon. That collision resurfaced the Earth and may be responsible for why the Earth still has a liquid interior, so much heavy material in its core, and so much water. See this article for some pictures of the simulations.

[Verginius Rufus]

There was a program on last night on one of the cable channels about this collision and how the moon was formed from the debris which bounced back into space.