Y

"Why" is the only question that bothers people enough to have an entire letter of the alphabet names after it.

The alphabet does not go "A B C D What? When How?" but it does go "V W X Why? Z."

"Why?" is always the most difficult question to answer. You know where you are when someone asks you "What's the time?" or "When was the battle of 1066" or "How do these seatbelts work that go tight when you slam the brakes on, Daddy?" The answers are easy and are, respectively, "Seven-thirty-five in the evening," "Ten-fifteen in the morning," and "Don't ask stupid questions."

But when you hear the word "Why?," you know you've got one of the biggest unanswerables on you hands, such as "Why are we born?" or "Why do we die?" and "Why do we spend so much on the intervening time recieving junk mail?"

Or this one:
"Will you go to bed with me?"
"Why?"

There's only ever been one good answer to that question "Why?" and perhaps we should have that in the alphabet as well. There's room for it. "Why?" doesn't have to be the last word, it isin't even the last letter. How would it be if the alphabet ended, "V W X Why? Z," but "V W X Why not?"

Don't ask stupid questions.

~ Douglas Adams,
The Master of all things Funny.