The thin curve of her lips was something of a perpetual smile, he thought. For even when her mouth was closed tightly, the lips pressed together and her jaw set in what was supposed to be rigid disapproval — of what, didn’t matter — that line curled slightly upward at the corners.

He thought a lot about that mouth: its expressiveness, and its playfulness. The way its shape changed when she was aroused was, itself, arousing. He loved watching it shift, slowly at first but with increasing speed, from simple full lips to an open oblong which increased, with short intakes of breath, to a large O.  As she rose rapidly to whatever new height he pulled her — or pushed her — it became a smaller o, a perfect circle.

When it was over, that taught little circle suddenly relaxed, collapsing and losing shape like a rubber band slipped quickly off the evening paper.  The rest of her followed, gradually, but it was always her perfect mouth that led the way.