You were invited into Sans's room, as you enetered Sans and Papyrus' house and upstairs as you enter Sans's room, as door suddenly closed and locked, which lead you to be locked in his room as Sans appears infront of you as he spoke before you even speak as he has very faint blue blush.

Sans: what? what does it look like i have tummy? that's cus I have one

His hands pulls out from his hoodie pockets and pulls up his shirt, revealing his blue ecto tummy where you see his ribcage

Sans: see? it's done of magic

You then reach your hands out and touch his tummy and touches it as you give it rubs and squeeze as he gets surprised

Sans: ah!?

He then tries to hide his belly/tummy as he tries to close his shirt, however you keep rubbing and squeezing his belly as he leans down as sweats runs down his skull while he seems to be shivering or shaking as he closes his left eyesocket

Sans: heeey... bit touchy there aren't we... don't stop