Try taking a tennis ball in your right hand, putting your thumb on the bottom and your middle finger on the top, and rotating the ball with the fingers of your left hand. The place where the thumb and middle finger of your right hand contact the tennis ball as it spins define the axis of rotation. The axis extends from the south pole to the north pole as it passes through the center of the ball. Over 1,000 years ago, explorers began using compasses, typically made with a floating cork or piece of wood with a magnetized needle in it, to find their way. The Earth has a magnetic field that acts like a giant magnet, and the compass needle aligns with it. The movement of the magnetic North Pole is the result of the Earth having an active core. The inner core, starting about 3,200 miles below your feet, is solid and under such immense pressure that it cannot melt. But the outer core is molten, consisting of melted iron and nickel. Heat from the inner core makes the molten iron and nickel...
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