"Y"
Mello was tired. The only thing he wanted to do was climb into bed and fall asleep. Unfortunately, he saw once he got in his room that that would be nearly impossible. It's very hard trying to sleep when you have a buffalo in your room.
'Matt!' His roommate came in. 'Why is there a buffalo in our room?'
'There isn't.'
'What do you call that?'
'A yak. Defiantly not a buffalo. Really, for a genius, how can you not be able to tell the difference between the two animals?' Matt giggled. Yes, that's right, he giggled.
'Why did you just,' Mello shuddered at the thought of saying the word, 'giggle?'
'It seemed appropriate at the time.'
'When is it ever appropriate?' The yak started chewing on Mello's dictionary. 'So, why is there a yak in our room?'
'How would I know?'
It was at this moment that a red headed boy walked in, closely followed but a girl with bushy hair. The boy pointed to the yak and said to the girl, 'Fred gave me a spell to turn it yellow! Want to see?'
Mello blinked. 'What are you doing in my room?!?!?! And why would you want to change him yellow anyway? That might make him mad, and then he will eat my entire dictionary!' He had just noticed the thing the yak was chewing, and now was trying unsuccessfully to pry it from the animal.
The red headed boy blinked. 'Well, I'm sorry, but your room was the only one that wasn't locked. If you don't want yaks in your room, you should lock your door.'
'You were the one who put this animal in my room?!?!?!?!' A vein appeared on Mello's temple. This was not a good sign. Usually it only shows up when someone is stealing his chocolate.
The red headed boy (who, if you didn't figure it out yet, is Ron from Harry Potter, and the girl is Hermione) took out a yo-yo and started playing with it for no apparent reason. Mello stared at it with a blank look.
'I love yo-yos. They're so fun. Can I see that for a moment?' Ron didn't give it to him, so Mello was forced to add 'Please.'
Once the yo-yo was in Mello's hand, he put it in front of the yaks face. The yak looked at it, and before Ron could protest, it had gobbled it up.
'My yo-yo! It was a magical yo-yo!'
'Magical, hey?' Mello's eyes turned to money signs, until he remembered that the yak had just eaten it. 'Dammit!'
So, he and Matt went to find another place to sleep while Ron and Hermione tried to get the yak out of the room. When they got back the next morning, it was to find that Ron, to get Mello back for the yo-yo thing, had painted their room… yellow. Poor Mello.