Where Endings Come From

Last night, Lupe posted this comment on yesterday's blog entry (I'm averaging one a day this week, although that will probably calm down once the book is finished and I don't need any excuses for not working, not to mention no more apartment flood crises, I sure hope).

Where was I? Oh yes, Lupe's question:

How do you visualize the ending? How do you know what the ending is going to be before you even know the middle of your book?

I've been frustrated by this everytime I write a story...I know...