Earth's magnetic poles have reversed hundreds of times over the last billion years.
Why it is true
These reversals are caused by changes in the flow of molten iron in the Earth's outer core. The last major reversal, the Brunhes-Matuyama, occurred about 780,000 years ago.
Source: Geophysics
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.