Why does Sans BLEED in Undertale?

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One of Undertale's biggest mysteries has always been the ending of the genocide route, where if you finally manage to hit Sans he will bleed instead of just turning to dust. This should be impossible considering the rules monsters in Undertale abide by, so what do you think happened?

#shorts
#undertale
#deltarune

[KazuNEET]

My head canon is that he just carries around bottles of ketchup.

[DrTrollolo]

"Aw fuck you sliced my ketchup!"
"And my ribcage... BUT MORE IMPORTANTLY MY KETCHUP!"

[lillii]

My headcanon is that Toby wanted a dramatic effect

[ItzRayCheesyRBLX]

Sans’ bones got cut, so the ketchup escaped from his belly

[dipadyck]

honestly Toby is the type of mf to be like “I just kinda forgot skeletons don’t bleed”

[arearon]

Probably ketchup or Sans and Papyrus aren’t naturally born monsters as we only know that they appeared out of nowhere

[NoahPhillips]

In Undertale, determination has a pure red color and ketchup has a very slightly brigher, less saturated color. If we do a color comparison, Sans' blood matches up to the hex code of ketchup.

[autumn_sunday]

My headcanon is that Sans wants you to feel bad. Also, in Deltarune, a monster is curious about blood...

[coffee_lver]

There's a few theories that people have come up with:
1. Sans was/is human/has a human soul.
2. He's hiding a bottle of ketchup in there.
3. He's come Deltarune.
4. Toby Fox is messing with us.
5. Sans is messing with us/trying to make us feel bad and reset.

[dr.coomerclone]

I’m pretty sure everyone has just universalized it as just being ketchup

[asinglesliceofbread]

Bone marrow manufactures blood. If you blow up an oil factory, oil spills.

[bloodcells]

People also ask
Can bones bleed?

Your bones are living tissue that can get bruised in lots of the same ways your skin can. It takes much more force to bruise a bone than it does your skin, but the injury is very similar. If something hits your bones with enough force, they can bleed without being broken.

[victorgout]

A french theorist that has really satisfying answers proposed that the red is not blood, but his determination leaking, as the only thing in color in the combat screens are the different types of soul, and the red is determination.

[KewlDewdPapyrus]

It’s because he stole my tomato sauce I was gonna use for my spaghetti

[user]

Toby fox: I forgor he can't bleed.

[gordonfreeman]

1. Bones create blood
2.he came from deltarune universe
3. Bottle of ketchup

[bjblgypshdharman]

He drinks whole bottles of ketchup and you're questioning why a red liquid comes out of him when you cut right through where his stomach would be.

[Vizenk]

He’s holding ketchup around in his pockets so when he got slashed the it kind of saved him, but he only had one HP so the ketchup bleed out like blood

[rfrx]

and what proves this that papyrus said that he and sans were living in a place that has green grass before moving to snowdin in the newsletter

[Undertalenjoyer]

It's not ketchup because the "ketchup" also comes out of his mouth if you hit him, and the only thing I think it is, it's a trick that Sans used to make you feel bad for all the things and reset and fix your decisions. He knows that it's you, the player that YOU did all the shenanigans in genocide. The "papyrus do you need anything" line is also approving this theory that Sans uses these as an emotional manipulation to the player.