The first time he tries to teach her how to dance is the last time, but she doesn't have it in her heart to tell him. They spiral and spin and River laughs brilliantly because she's always known that he's got two left feet and isn't in any position to try and tell her how it's done.
"You're doing it wrong," she says as they stumble.
The Doctor grins, wide and amused.
"I know," he tells her. "I'm rubbish at this."
They're on a planet where everything is cast in purple light like an everlasting sunset, and the ground is made of rocky crystals, not at all suitable for dancing on. She's almost sprained her ankle once and she's lost count at how many times he's stepped on her feet. River isn't sure that she cares though, because if this is going to be the last time it ever happens she wants to make sure it's the very best it can be. She always tries to make sure that the Doctor's firsts and her lasts are the best they can be because they're the memories she's going to be left with to hold onto one day.
"Who taught you how to dance so well then?" he asks.
She's tempted to just say spoilers and smile, but this time she doesn't because it's not as important.
"You did, sweetie." River says, and it's true.
It doesn't really make much sense that he taught her how to dance based on what she's teaching him now, but sometimes you can't argue with sense. The universe is complicated and doesn't feel the need to justify itself too often, which is something she's learned from experience. They've danced many times before, and he'll dance with her many times in the future to come. The last time they did something like this together was on her birthday, and the Doctor was a little better at doing it then than he is now. Right now, though, he's got no sense of rhythm at all, and River isn't sure that he's even trying to do this seriously anymore.
The best time they ever danced was somewhere in the middle of their timeline, when they both knew just about as much as the other about each other's futures. River wasn't clueless about him, and the Doctor wasn't clueless about her. He was alright at dancing and so was she, but he was still as ridiculous about the whole thing as ever. They'd tripped over their feet a little bit and he'd spun her exaggeratedly and laughed when she said something witty, but at the end of the song he'd pulled her in and kissed her soft and sweet.
The next time River asks the Doctor to dance, he barely knows who she is.
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