Sans eyed Frisk with amusement.
"You want to know how skeleton anatomy works?" Sans demanded.
Frisk nodded rather eagerly. Ever since Mettaton and Papyrus had begun dating, the questions had been mounting rapidly. It was straightforward on Mettaton's end of it. He was a humanoid robot with what Frisk assumed was a very through design. However, Papyrus was an entirely different creature. As Sans had promised to give Frisk anything they wanted for their eighteenth birthday, Frisk decided to take advantage of this to ask a few questions.
"You know skeletons have magic, right?" Sans asked.
Frisk raised their eyebrow, but nodded.
"Well, by the laws of your world, we shouldn't even be able to stand up," Sans pointed out. "But we're monsters, so those laws don't apply."
Sans stretched out on the couch with a sigh. He normally didn't like explaining things, but he was willing to make an exception for Frisk.
"I don't know what I could really tell you about our anatomy," Sans continued. "We're not..."
A realization struck him.
"Ah," Sans uttered slowly. "You want to know about our...things?"
Frisk nodded again, albeit reluctantly. This was where the conversation had been destined to end up, as intrusive as they realized it was. But they thought Sans was the type of monster who wouldn't object to intrusive questioning.
"It's not too complicated," Sans responded with a shrug. "When a skeleton gets aroused, they summon what we call an "ecto-dick". It only exists as long as it needs to, though. And you can't force a skeleton to summon one."
Frisk frowned. They'd suspected it was something along those lines, but Sans' explanation had a very obvious hole in it.
Sans chuckled.
"You're wondering why we'd need to do that, right?" Sans guessed. "We skeletons don't reproduce the way humans do, so why would we mimic human anatomy?"
Sans adjusted his position on the couch. He was starting to enjoy himself a little.
"Truth is, it's just a coincidence," Sans explained. "Ecto-dicks look like that because it's the most convenient shape for their purpose. Notice how skeletons have a lot of holes in their bodies? Yeah, you can see where this is going…."
Frisk tilted their head to one side, examining Sans's face. They thought he looked a bit less relaxed than was normal for him.
"So why do skeletons even need sexual attraction?" Sans continued. "That shouldn't exist for us, right? I mean, you humans only want to bone each other because of your natural instinct to create tiny versions of yourselves."
Sans looked at Frisk pointedly, as if indicating them as proof. He occasionally forgot that Frisk was a human. He preferred not to think about humans and their baffling mating rituals. Although he supposed he should be thankful such rituals existed, otherwise Frisk would not.
"For skeletons, we don't just bang because we can," Sans explained. "Sex is...well…kind of an important thing to us. It's how we bond with our partner both physically and mentally. The reason it feels so damn good in the first place is because we're absorbing each others' raw affection."
Frisk furrowed their brow for a moment. The information was appreciated, but there was one thing in particular they'd been wondering about. To demonstrate their query, Frisk pretended to fan their face. As they expected, it took Sans a minute to understand.
"Yeah, we go through heat," Sans answered with a laugh. "That's the technical term for it. I call it "being bone-ly". Skeletons don't do too well without special companionship. Once a month we just go a little crazy with our desire for it. It's pretty bad, but unavoidable. We just have to stay in the house and wait it out."
Frisk gestured towards Sans with a raised eyebrow.
Years of dealing with Frisk's silent communication instantly came into play. Sans realized within a second what Frisk was asking. His cheeks flushed bright red.
"Have..have I ever boned anyone?" Sans asked. "N-No..."
Sans tried to laugh off the question, but his mirth was forced. His mind wandered to Mettaton and Papyrus.
"Nope, never been taken to the Bone Zone myself," Sans uttered, shrugging. "But why would I want to? That sounds like work. And you know how busy I am."
Sans pulled the hood of his sweatshirt over his head. He laid his hands across his ribs, settling in for another nap. He was about to drift off when Frisk shook him, startling him back into reality. He sat up quickly with a groan.
"What is it now, kid?" Sans asked, although not unkindly.
Frisk unhooked the bracelet they were wearing around their wrist. It was a special gift from Toriel. Dangling from the bracelet were seven heart-shaped charms. Toriel claimed jewelry and the like containing hearts were lucky for monsters. Recalling this with a smile, Frisk gently took Sans's wrist in their hand and carefully hooked the bracelet to it.
Sans stared at his wrist for a minute, then looked at Frisk's face. It was often difficult to decipher Frisk's expression, but Sans thought they appeared sympathetic.
"You're...giving me this?" Sans uttered slowly. "You...think it'll help me find my special someone?"
Frisk nodded.
Sans's eyesockets filled with tears. He hastily wiped them away, but he knew Frisk had seen. Somehow, knowing this caused him little embarrassment. Sans's wrist dropped to his side.
"Thanks," Sans uttered gratefully.
Frisk perched on the arm of the couch tenderly stroking Sans's skull until he fell asleep.