He's always been a very observant, so he knew from the first moment you stepped foot in his house, something in your way of being had changed, so, from the first moment you stepped foot in his house, he knew that something in your way of being had changed.

The evening you spent with him and Papyrus was as normal, but he kept stealing glances at you everytime he turned to look at you, but it was more like he was catching you staring.

Papyrus went with Toriel and Undyne to their new kitchen in the surface and left you two alone.

He took the opportunity to ask you.

"you've been staring at me all night," he noted, calling yout attention.