Why isn't Pluto a planet any more?

For 76 years, every textbook said the Solar System had nine planets, and the ninth, far out in the cold and dark, was little Pluto. Then in 2006 the world's astronomers voted, and suddenly Pluto was off the list. Children who had just memorised nine planets were told there were eight. But Pluto didn't move, shrink or vanish. Nothing out in space changed at all. So how can a world stop being a planet?