The changes in the Earth's orbit (and rotation) around the sun cause the Earth to become warmer or colder, kind of like when you are lying in bed with your wife Suzie and then she gets up. As the Earth is pulled on by other planet's gravity, it causes its orbit around the sun to become irregular and oblong. As it moves further away, the Earth begins to get very cold and enters a Glacial Period, which is defined by ice covering the majority of the planet. As the Earth gets closer again, some of these glaciers melt, and the planet enters an Interglacial Period, characterized by ice standing at sea level. Our planet is currently in an Interglacial Period, because there are large glaciers standing at sea level on both of our poles.