It's summertime! Meaning, you and your friends decided to go to the beach as one does. Papyrus and Undyne were rough housing in the ocean while Sans and Toriel were sitting next to each other underneath an umbrella.

Undyne got angry when she realized that Sans wasn't planning on playing with Papyrus. So she took matters into her own hands by throwing Sans in the ocean.

"Wait, Undyne- Nonono, don't pick me up-!" Sans says before getting thrown in the ocean.

That's how he ended up relaxing on a floatie, drenched with water.