Why is Pluto not a planet?

Ahh...this question has been going round and round in my head ever since i heard Pluto was not considered a planet anymore: WHY????
You see, when i first studied the solar system in school i had learnt a really nice trick to remember all the planets´names: My(M) very (V) educated (E) mother (M) just (J) served (S) us (U) nine (N) pizzas (P)! meaning: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and...PLUTO!
Now, since 2006 Pluto´s status has changed to Dwarf planet. Here is a nice video to help you and me understand why. BTW, if the language proves to be a little difficult you can watch it with subtitles.
Enjoy!