The Seventh Element Of Harmony

News from Princess Celestia, Twilight P.O.V

While I was reading, I heard a noise, a burp. Ah! A letter from Princess Celestia, but what did she want? "This is for you." Spike said as I grabbed the letter.
DEAR TWILIGHT
I HAVE JUST BEEN INFORMED THAT THERE IS A SEVENTH ELEMENT OF HARMONY!
HER NAME IS MOONLIGHT DREAMER AND SHE WILL BE ARRIVING IN PONYVILLE TOMORROW. SHE WILL BE LEARNING THE MAGIC OF FRIENDSHIP WITH THE HELP OF YOU AND YOUR FRIENDS.
I HOPE THAT SHE WILL LEARN LOTS FROM YOU AND YOU WILL BECOME GOOD FRIENDS.
YOUR LOVING TEACHER
PRINCESS CELESTIA
What? A seventh element of harmony? That's impossible! How?
"What's wrong Twilight?" "There's a seventh element of harmony and she is coming tomorrow! I need to help her learn the magic of friendship. But why Spike? Why is there a seventh element of harmony? I just don't understand."
I looked through my books but there was nothing about a seventh element of harmony. Oh well, I guess I'll just have to see myself tomorrow when she arrives.