Board Thread:General Discussion/@comment-31270796-20170326200357/@comment-44609834-20191207030144

Burgrr wrote: 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). He probably either got bored waiting for you and had a snack, simply eats it for his meals (likely) or wanted to screw with the player. Hell, he could have been holding ketchup in his mouth the whole battle and planned to spit it at you, who knows with him?
 * i'm just responding to the bolded parts

-Yes, current humans can't use magic, but what about the ancient ones? We already know at least seven (likely more) could. And with Determination and magic combined, who knows what they could do to themselves to get into what seems an almost dead state (1HP, just a skeleton, etc)

-Don't monsters just absorb their food? Is it even possible for them to cough it up?

-He speaks to you quite a bit, so I doubt that's the case. It would be difficult to hold something in his mouth (or even put something in his mouth in the first place - can he even open it?)

Personally, I love to have more fun ideas than the obvious when it comes to theories. Yes, it is most likely to be bone marrow containg blood or the ketchup theory, but not only are they the obvious, they can also be disproven just as much as any other theory.

Of course, everyone is entitled to their own theories and opinions. I personally really enjoy either the "former human" or "gaster and/or determination experiment" theory.