Sans: //chuckling, has usual grin// 

Frisk: Sans, why are you always smiling?

Sans: //smile strains// I told too many puns and my face froze like this. 

Frisk: ... MOM I CHANGED MY MIND ABOUT THE COMEDIAN CLASSES!

Alternative: 

Sans: I told too many puns and Papyrus dumped one of Alphys' potions on my face and it froze like this.

Frisk: .... MOM I WANT A RESTRAINING ORDER