The answer: because Rowling said that if she had a patronus, it would be an otter, just like Hermoine's.
I've tried to give you guys hard ones, so each will be worth 5 points. You have until next monday to do it.
1. What day was Neville born on?
2. What is interesting about the names of all of Sirius's family members (including himself)?
3. What is Hermione's patronus?
4. Why is Hermione's patronus interesting?
5. Who tested Dumbledore in his N.E.W.T.S.?
6. When Rita uses the name "long-haired pillock" in an article, who is she talking about?
7. What type of wand did Hagrid have?
8. What is Ron's middle name?
9. What is Dumbledore's full name?
10. What is the date of Draco Malfoy's birthday?
EDIT: I did some major typos, but they're fixed now. Whoopies.