As you arrived to Snowdin Town, you noticed a familiar figure going in of a bar called 'Grillby's'. It was Sans.

You come after him and when you got in, you saw how Sans interacted with the other monsters, you could see how he's famous. You approached him and sat next to the stool of his.

He noticed your presence and smiled. "oh kiddo, what'cha doing at the bar? aren't ya supposed to go the Waterfall?"