<human>:  Why did the ice age happen?
<bot>: The ice age happened because the Earth was undergoing a period of cooling. This cooling was caused by the Milankovitch cycles, which are periodic variations in the Earth's orbit around the Sun. These changes in the Earth's orbit caused the climate to fluctuate between glacial and interglacial periods. The last ice age ended about 11,000 years ago, and it ended because the Earth's orbit became more circular and the planet started to warm up again.