Sans the skeleton. Not my personal favorite character, but definitely a fan favorite. Known mostly for his puns and unexpected boss battle in the Genocide Route, this floofy skeleton has gotten fans going wild to see more of him in fan animations and fanfics. But is there more to him than meets the eye?
In Undertale, there are lots of things that don't logically make sense, but are simply there for the sake of the plot and comic relief. I don't need examples because I'm pretty sure you Undertale fans know what I'm talking about.
But Sans seems way more different than all the other characters. Though he's not my favorite like most fans, he's rose speculations and curiosity into my head. I'm gonna stop saying my opinions and stuff and get straight to the point:
Is Sans human?
I know what you Sans-lovers are thinking. And I know you're about to exit this book. Do it if you want to, but I think my evidence will make you wonder the same thing as well.
Is Sans' face actually a mask?
To start with, Sans' mouth does not move while speaking. Most characters do that, but this is obviously an exception. He also has pupils while his brother, Papyrus, does not... Although this may just be over-thinking things since Papyrus has smaller 'eyes' than Sans and he'd just look goofy with little white pupils. As for Sans, imagine him without pupils. That'd give players the feeling that he's 'creepy' because generally characters without eyes turn out to be an antagonist(this only happens with Sans during the Genocide Route). With his pupils, Sans looks much less threatening and even a bit friendly.
On the topic of eyes, some people may say that if Sans really was wearing a mask, would he be able to wink or close his eyes? No, but in rebuttal, neither can a skull. In the game Papyrus can move his eyebrows while in dialogue to give him expression (one of those things for the sake of the plot) but I don't recall a time when he closes his eyes.
Let's move on to Sans' apparel. He's definitely wearing a hoodie, right? But it's not confirmed that he's wearing a white t-shirt underneath. Looking at his sprite in battle, there is a white line in the center of his torso, and coming from the sides seem to be branches. To some people, this is just a shirt and the teeth of the hoodie's zipper, but they are more likely to be ribs. The size of these branches seem to be a bit large and over exaggerated for them to be a zipper. Let's take a look at his other sprites. When standing to the side, you can see the ribs on his torso. So this is not a shirt: these are ribs.

Picking up from Sans' overall appearance, a noticeable trait about him is that he's fat. How can a skeleton be fat? Can he just have big bones? Is he storing ketchup packets and bottles under his jacket? I'd say one would get chubby from eating too much ketchup, but if Sans is just bones, then it's impossible to gain weight.
Ok, if you made it this far, thank you! By the way, keep in mind I'm also kind of shunning my OCs-- I have a skeleton OC, Dax, who had children at an early age. ((NO. Not by Sans and Papyrus. She's their sister.)) But, like I said, some things are to be just for the sake of a plot, but Sans' size doesn't seem to be really important to any of the routes to me. Alphys could be tall and skinny rather than short and stout, but that can't change who she actually is. Okay, now I'm confusing myself, but please stay with me.
In the Genocide battle against Sans, if you survive all the way through the weary 10-minute-battle, you finally have the chance to kill your opponent. But when you slash the dagger across his chest, he begins to bleed.

Wait a sec. How can a skeleton bleed?? I understand that the knife could break a few ribs, but... Wuuuutttt?? Papyrus doesn't bleed when the player decapitates him. So why should Sans? Some fans claim that the 'blood' is actually ketchup, but do you remember when I told you about the ribs? If he stuffed packets inside his ribs, then they should be visible when looking at Sans face-to-face. Plus, if it was ketchup, then the spine would also need to have red spilled on it. How would the 'ketchup' be spit up from his mouth if it was stuffed in his jacket? The only way one could spit up blood was by the blood travelling up their throat, but skeletons don't have that, now do they?
Nope!
Also, thanks to CINNAMONBUNPAPYRUS15, I remember that Sans also has a secret hatred for humans; but maybe this is because of his past or just for the sake of keeping his secret identity secret.
Okay guys, I think I'm done. If you have anything to add on or you would like to argue, please go to the comments. ^o^ I'm sorry about all this. I know how a simple theory could ruin a character for someone, so I'm sorry if you feel that way right now. But hey; even if Sans is human (which isn't confirmed by Toby Fox or any of Undertale's creators), then it wouldn't change any of the game. It wouldn't change a sequel either, but I doubt there would ever be a second Undertale for obvious reasons.
Ok I'm done. I hope you enjoyed!!