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PLANET GENERATORS
A magnetic compass (not the kind used for drawing circles) can detect a very small magnetic pull. The compass needle usually points north because the planet you are standing on is a gigantic magnet. The Earth's magnetism is produced because its melted insides spin at a slightly slower speed than we do on the surface. The outer crust acts as stator and the inner core acts as the rotor. This electricity creates a slight magnetic force.
NOT SO CLOSE TO HOME: When the interplanetary probe Galileo passed Ganymede (the largest moon of Jupiter) in July of 1996, it found that it had a magnetic field. This is evidence that the moon has a liquid core.