Early on in the earth's formation, when it was still nothing but a lump of semi-molten stuff, a smaller, rogue mass of semi-molten stuff crashed into it. The force of the impact organized the earth's matter into layers--inner core, outer core, mantle, crust--and set our planet's axis on an angle. A portion of the rogue planet spun off from the earth into lunar orbit. Still today, the moon keeps the earth steady on its tilted, season-making axis.