And a last.
His eyes are so blank, she thinks he must be playing her, teasing her before he wraps her in his arms and gives her the kiss that can melt the toughest archaeologist in the universe. Nine hundred years is a long lifetime to kiss and be kissed, and he's made good use of it. The first time, she felt like she was drowning. He only smiled and winked, as if they'd done it a thousand times before. She didn't know then that they had.
She can handle him not knowing, keeping her off balance with the things she remembers that he doesn't know yet. Spoilers. It's all right because their relationship is more than that. She likes being a mystery, and he likes the guessing game. She doesn't mind being his future. She likes knowing that when she's gone, there will be more for him to experience, more happiness than he can imagine. It's all right that he gets younger as she gets older. She gets to teach him to love her, the older River making the Doctor ready for the young girl he'll one day meet. She can handle whatever this back-to-front relationship throws at her.
Until today.
It's no less enjoyable. She feels his lips, smells his scent, lets herself relish the closeness that will carry her through until she sees him again. But it's different. He's different. Uncertain and a little bit clumsy, like someone who remembers how to ride a bike but hasn't done it in ages.
She steps back and looks at him, and she realizes. Today is the last time. Today is the day she can't handle.