How do the leaves know when to fall?

I was sweeping the sidewalk today, and I noticed some unnerving evidence of fall coming. There were leaves on the ground. I'd been hearing the acorns hit the roof for some time, but leaves are a lot sneakier. They don't announce their fall with a loud rap you can hear from below. They just...flutter.

Thinking of the end of summer with a pang reminded me about endings in general, especially since I've been struggling with one on a picture book. A good ending is like the leaves falling from the trees. Even though you may not want the story to end, it feels right to end it at that point.

What are your favorite book endings? Do you find that it's a bittersweet time that you want to go on, but know that it's over? Do you ever feel cheated at the end, like the author missed her timing somehow and ended in the wrong place?