This was just a silly thing I came up with.
Frisk Feeds the Skelebros a Truth Potion
Late in the evening, a tiny human far too curious for their own good slipped into the Skelebros' kitchen. A pot of spaghetti bubbled on the stove. Giggling, they carefully poured in a few drops of the clear, odourless liquid right into the pot.
(At last,) thought they, (I will have answers! Why do they have that mysterious lab? What does that broken machine do? What are they hiding? Why does Sans mess about with those dumb passwords and what does Papyrus REALLY know about time travel? Why did Papyrus pretend to not know what a lab was in front of Sans?) It mattered not. They WOULD find out!
But just as Frisk was at the table with Papyrus and Sans, about to ask the first secret-revealing question, Undyne showed up. Which would have been fine, had she come to ask about literally anything other than
"Oi! Sans! Why'd you turn your sentry station into a freaking HOT DOG STAND and-"
"needed the extra pay to make the rent." Okay, not THAT question. It was the NEXT one that really set him off.
"Yeah, I knew that, I mean what's it doing IN THE MIDDLE OF THE ROAD?!"
By the moment Frisk and Papyrus had turned to face him he'd already started talking.
"oh yeah. that. well, first i took the original sentry station and..."
That was about as much as any of them could understand before he launched into a detailed monologue about the exact physics behind how one sentry station wound up in four different places at once, along with the exact miscalculation that had resulted in the station being stuck to the middle of the local road.
"...and i'm too worried about the possible consequential imminent implosion of the universe to bother attempting to fix at the moment. gimme five minutes after dinner and i'll have it sorted, k?"
"Uh... What..?" Poor Undyne looked like someone had bashed her over the head with a hammer.
"what? wait, why did i just say all of that." Sans gave Frisk a suspicious look -guy seemed to think that everything that went wrong was their fault somehow- and added, "y'know what? i'm overdue a very long nap. nobody talk to me." And he was gone from the table before Papyrus could even think to complain.
Unfortunately, Papyrus was apparently a lot smarter than people gave him credit for. "OH NO! AN UNCHARACTERISTIC QUANTUM PHYSICS-BASED MONOLOGUE! QUICK, UNDYNE, ASK ME SOMETHING I'D NEVER ANSWER HONESTLY IN A MILLION YEARS!"
"Uh... Okay? Hmm..." Undyne mulled over it for a few moments. Frisk knew exactly why. After all, since when was The Great Papyrus anything but "one-hundred percent honest" at least as far as she was concerned, well, not including anime opinions?
"I KNOW!" She leapt up, ponytail nearly touching the ceiling as she dramatically pointed at Papyrus's face, "What to you REALLY think about your brother's infamous puns?"
"THAT THEY'RE ACTUALLY KIND OF... CLEVER. AND IT IS VERY RELIEVING TO ME TO SEE THAT HE DOES STILL CARE ABOUT SOMETHING HE USED TO LOVE DOING!" There was a slight pause as Papyrus thought over his own answer. "OH. I'M SORRY ABOUT THE STATION UNDYNE, BUT UNFORTUNATELY I AM FEELING THE SUDDEN NEED TO GO AND HIDE IN MY BEDROOM. PLEASE TO NOT TALK TO ME UNTIL WHATEVER MAGIC GOT IN THE FOOD WEARS OFF."
He'd gone all the way up the stairs by the time he was done talking.
Frisk decided that they really didn't want to stick around to see the consequences of this disaster. They loaded their SAVE, vowing to get the answers someday no matter WHAT it would take!
THE END