Why do planets rotate?
All particulate matter in the universe has angular momentum (called spin at the subatomic scale). As I understand it, the angular momentum of a macroscopic object is basically the sum of the spins of all its constituent particles. Another way to look at it is to note that when solar systems form, the original disk of dust is spinning. If it wasn't, it would all collapse into a single large star. As portions of the disk accrete into the planets, some of the overall rotation is translated into a rotation of the planet because angular momentum is conserved (angular momentum is a function of the distance from the pivot point, so as that distance is reduced, it translates into more rotation instead). What's left keeps the planet orbiting its sun. Hope that explanation made some sense.