Undyne tried to get Sans to spar with her. -- TheDragonsFlame
Sans slept like the dead. If you'd pardon the pun, because he was in fact a living skeleton. Only one thing could wake him up and that was the smell of ketchup on a warm hotdog. He leaned towards it, only to have it lift away. By the time he was sitting up, his eyes were open and he could see Undyne with one of the best hot dogs in the world, tantalisingly out of his reach.
"I heard the human say you were one of the toughest fights they'd ever experienced," she said. "Though I can't imagine that weakling ever fighting anyone."
Unfortunately, Sans could. This was a human who had started without mercy and gone through two more iterations where they fixed everything. He remembered that fight. And he remembered the apology in the human's lidded eyes. They were sorry enough about what they did to reset, and let the underground live. And reset again to make sure they were all freed. "Yeah, sometimes that kid has a weird imagination," said Sans. "You know I don't fight."