Can I use a quotation for the title of my essay?

Yes, you can use a question to title your essay. I've written several articles and short stories that open with a question, but I always included several possible answers (where there is a definite flexibility to the work e.g.: debating the pro's and con's of wearing mis-matched socks). Where there is only one answer, such as a fact, historical fact, mathematical – anything like that – I get down to the nitty gritty almost immediately. However, it's vital to not answer the question withing the first two or three sentences! I discovered that when I provided the correct answer immediately, people tended to just scan the rest of the story.