Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: Alisha's teacher glued a needle to a piece of cork so that the needle would float. Then he used a magnet to magnetize the needle. When the needle was placed in a bowl of water, it turned to point north. Which best describes the reason that the needle pointed north?
Earth's magnetic field.