Board Thread:General Discussion/@comment-44714172-20191224031018/@comment-26907577-20200104200139

Look, magic can't explain everything about monsters. The base fact is that a skeleton which is animated by magic has no reason to bleed due to magic.

So, either a) Sans isn't a skeleton animated by magic, he's just pretending to be, or b) it's not blood, not real blood anyway.

Option (a) has sub-theories such as "Sans is a Darkner" and "Sans is a human".

Option (b) is prooooobably either ketchup or fake blood.

And, if Sans set up the scene just right, he could totally fake the blood using ketchup. One last, nonverbal, "think about what you've done and please don't destroy the timeline".