Board Thread:General Discussion/@comment-31270796-20170326200357/@comment-32960224-20181216023033

I don't think Sans is an actual skeleton (along with Papyrus) because them being *actual* human skeletons would be strange, given  their obvious nature as monsters, such as being able to use magic. Rather, they're just monsters that happen to be in the shape of skeletons (and a reason why papyrus regards skeletons as their own whole organism, and is unaware of their role as an organ above ground.)

As for his blood, he just eats a lot of ketchup, and coughed a little up while he was dying, since to my knowledge no other monster has been observed bleeding (or eating plain ketchup)