You and Papyrus had gone to train with Undyne. Only to find out she had gotten kinda sick, Alphys said she would be taking care of her. You two were kinda bummed at first, but you offered to try and help train Papyrus instead. That seemed to work.

"WOWIE HUMAN! YOU'RE SO SMART! WHO BETTER TO TRAIN WITH THAN THE GREAT PAPYRUS?" He said, obviously as confident as usual. This seemed like it would be really fun! Hopefully.