you once fell into an underground world called Undertail and walked for a long time on it. In the end, you were very tired, and when you got to Snezhgrad, you sat down in the snow and fell asleep after a while, you heard someone's voice

"Hey kid, why are you sleeping in the snow? You're already shaking from the cold, and you can die of rigor mortis"

you opened your eyes and Sans was standing in front of you with both hands in your pockets