Disclaimer: In this story line, Papyrus and Sans are raising a 15-year-old Frisk.
You and Sans sat in the living room of your house in Snowdin, and you drew pictures of all your family and friends. You hummed a little song, as Sans was dozing off on the couch. You enjoyed your time with the two skeleton brothers, and they always seemed to feel the same way. After first coming the Snowdin and stopping Papyrus from fighting you, you all decided that it would be for the best if you stayed with them for a little longer. So, now you spent your days going to work with them.
Nights were usually spent hanging out with Sans like this, while Papyrus trained with Undyne. It was always cold here, so they had bought a large bag filled with various blankets for you within the first month of you arriving. They never needed them, considering they were skeletons, so most days you were huddled in twenty or so of the soft warmth of the blankets and spending time with your new family.
Papyrus liked teaching you, but sometimes you got really tired from his constant lessons of cooking and fighting. You never really liked the fighting, though, so you would stand their with a few blankets draped over you as he told you how to block attacks instead. Sans taught you the best pranks and puns, and also some various scientific theories if he was feeling particularly energized. Papyrus didn't appreciate the two of you both starting a prank war with him as the main target, and currently you were in the lead by one.
Today, Papyrus came home early, bounding with even more energy than usual. You ran up to great him and exchanged a hug with the taller of the skelebros, and he started going on and on about his apparently fantastic day. Sans was pretending to be asleep, snoring loudly, with his usual grin plastered on his face.
"Frisk, I'm so excited! Undyne told me that she'd ask Asgore if I could help her do paperwork for the Royal Guard for her! That means I'll be practically in the Royal Guard!" He exclaimed loudly. Sans appeared next to you both, yawning loudly. You couldn't help but also yawn, and he winked at you.
"Geez Papyrus, calm down. Don't get a bone-r about it." He said, raising his eyebrows and snickering, searching for a reaction from Papyrus. He stomped his foot angrily and looked upset at the pun.
"Uhm... what's a boner?" You asked curiously, tilting your head. The two of them blanched and exchanged uneasy glances, unsure of what to say. Neither of them answered, and for the next hour or two, it was absolute awkward silence.
After Papyrus made a suprising meal of spaghetti for dinner, you all sat down in the living room and ate, still in silence. Sans was emptying a little packet of ketchup onto his spaghetti, much to his brother's dismay. Awkwardly, Sans cleared his throat and looked over to Papyrus, trying to motion for him to say something. You looked back at them both and tilted your head in confusion again, wondering what they were trying to say.
"Uh. Well, back to your earlier question, Frisk..." Papyrus said, and trailed off. His brother picked back up.
"Well, a boner is something that happens when someone... is what happens.. when you get excited?" He looked more uncomfortable than you had ever seen him.
"Oh... so when a dog wags his tail, that's a boner?" They both choked on their dinner, and Papyrus stared at you, his mouth wide open in shock.
"No! A boner.. is when a penis get hard." Sans said, trying to finish the sentence as quickly as possible. He stared at the ground and avoided looking at you. You turned your gaze to Papyrus, and he cleared his throat.
"What's a penis?" You asked. "And what do you use it for?" The nervous air in the room made it almost unbreathable, and they grew silent again, hoping the other would speak up so they wouldn't have to.
"Well, every monster has special... parts... that they use to... make new monsters. You see, when a daddy monster and a mommy monster love each other very very much, they-" Papyrus started to say, but was cut off by Sans panicked screaming.
"I DON'T THINK WE SHOULD BE TALKING ABOUT THIS." He yelled, his voice seeming to be a higher pitch. You looked confused, and stood up, putting your dishes in the sink and frowning.
"Should I look it up on Papyrus's computer so you guys don't have to explain it?" Yawning slightly while you said so, stretching.
"NO THAT IS CERTAINLY NOT A GOOD IDEA DO NOT DO THAT." Papyrus said, also in a panic.
"Listen, Frisk. Listen. Alright. So, everyone has private places that they don't show other people, you know?" Sans tried to explain to no avail.
"Like... Your room, Sans?" You asked innocently. They both blushed their respective hues.
"NO. Like. On their body, they have places that they don't let people touch. Some of them have penis's, some have other things. When a monster with a penis gets excited, their... penis... gets hard... and they call that... a boner..." Sans spoke, gradually getting quieter and blushing even more, choking out each word painfully.
Much to their shock, you doubled over, howling with laughter. At first, they thought something was wrong, and both stood up from their seats to make sure you were alright, but once they realized you were laughing, they exchanged horridly confused glances.
"W-what's so funny, kiddo?" Sans stuttered. You kept laughing, eventually with tears flowing down your face, as you tried to take deep breaths in order to stop laughing. They looked even more confused at this, and you eventually contained your giggles.
"I tricked you guys!" You stood up, stretching. They both blanched again, and so you elaborated. "Guys, I'm not that naive. I knew what a boner is. I just thought it would be funny if I pretended I didn't! I pranked you both!" You gave a triumphant laugh, and Sans grinned with pride, but he was still blushing. Papyrus also was blushing, and he tried to look angry, but he was also amused by your stunt.
"Wow.. I haven't been tricked like that in years. Good job, kiddo, looks like I've taught you well." The shorter skeletal brother proclaimed with a wink. Papyrus still seemed to be in shock of being duped that extensively.
"Good job on the Royal Guard stuff, by the way!" You said to him, and he looked a little better.
You walked up to the both of them, and gave them each a hug, and curled up onto the couch with your piles of blankets and went to sleep. You were proud of yourself.