Here's why:
* Definition of a Planet: A planet is a celestial body that orbits a star, is massive enough for its own gravity to make it round, and has cleared its neighborhood of other objects.
* Definition of a Moon: A moon is a natural satellite that orbits a planet.
While some moons are very large, like Ganymede (Jupiter's moon) and Titan (Saturn's moon), they are still smaller than the smallest planets in our solar system, Mercury and Pluto.
