There is only one day out of the entire year that Sans wakes up early. This day happens to be April Fools.
*Yawn* Sans stretches as he gets out of bed. A few of his bones pop but he doesn't care he has to hurry and set his little 'surprises' all around Snowdin before anyone wakes up. "This should be fun" He says with a menacing smile on his face. It's one of those smiles that when in school his teachers would exclaim 'Don't even think about it' before Sans could do anything. Don't get him wrong he plays pranks on people all the time but when it's April Fools, he lets loose his best of the best and this year he's aiming to get back at Grillby for last year.

~The Year before~

Sans smiles as big as he possibly can when he walks up to Grillby's restaurant immediately after walking in people start cowering in their seats not
wanting to get caught in his wave of never ending 'tomfoolery' as Papyrus would put it.
If it were possible he would smile even wider as Grillby turns to him as if it were any other day and asks "... what will it be... Sans?" Sans smiles as he jumps up on his stool. A woo-pie cushion goes off but he doesn't mind
they're his anyway and that might explain why no one was sitting up at the bar when he got there. Even drunk bunny was sitting at a table and he always sat at
the bar.
He turns to Grillby with that wicked smile on his face and says "just the usual Grillbz." While Grillby goes off to get him his food Sans begins to unscrew the caps of some of the condiment containers laying on the counter. He snickers when he finishes just seconds before Grillby gets back with a bottle of ketchup
and a cheese burger with fries.
Sans smiles up at him innocently. "You know Grillby you are one HOT waiter. Grillby smirks at this as he sets the plate down in front of Sans and hands him the ketchup. But before he turns around he begins picking up a conversation with Sans.
"...So just a question but why do you drink ketchup?"
Sans looks up at Grillby and shoves a few salty fries in his mouth before answering.
"It taste good." As if to prove his point he downs half of the bottle but immediately regrets it.
He looks at Grillby who just starts laughing and everyone in the bar joins in when Sans begins to cough and gag. He looks to Grillby mystified that even Grillby could do something like this, it's the first time he has ever known Grillby to prank anyone and least of all him!
"Grillby how could you that wasn't COOL!" He exclaims still coughing. His non-existent throat feeling like it is on fire and Grillby is by this point doubled over on the floor laughing so hard while his flames crackle every which way with his amusement. After managing to compose himself Grillby went off to get Sans a drink. He comes back with a glass of water managing not to spill any on himself as he hands it off to Sans still snickering under his breath.
Sans downs the drink and is relieved to find that it's just normal water. His throat feels better now and after a few seconds he finally is able to talk.
"What was that?" He asks after clearing his throat so it doesn't come out hoarse from coughing so much.
Grillby finally settled down and turned to him as if nothing had happened at all.
"...What," Grillby asks "do you not like my homemade hot sauce?" He begins laughing again and everyone joins in once more while Sans puts his hood up and walks out of the bar
his face bright blue from embarrassment. He vowed then that he would get Grillby back the next year and that is exactly what he planned to do.

~Present Day April Fools~

Sans quickly walks around the house and puts some of his little 'surprises' out for Paps. He knew Papyrus would probably just leave to Undyne's as soon as he woke up
so he wouldn't have to deal with Sans's pranks but he could never be too sure if Papyrus would forget and sit of the couch or something.
After placing Woo-pie Cushions under the couch cushions, placing rubber chickens under the door mats, and putting coiled spring snakes in the refrigerator. Sans heads off to the labs.

~ A Month Before~

Sans teleports into the labs after not getting an answer from Alphys at the front doors. He knows he should probably be on watch with Papyrus to stand guard for
humans but he needed help to get back at Grillby for the year before on April Fools. Besides Papyrus would not go back and check on him for at least an hour so he was
fine. He looks around but everything is dark.
He goes to flip on the switch but as he does he hears a squeak of surprise from Alphys who was at the monitors eating instant
noodles in front of a laptop watching anime.
"S-Sans! J-jeez! You sc-scared me!" She stutters out before pausing her anime and turning to face him.
"sorry Al," he apologizes, " i rang for ya but you didn't answer"
He walks over from the light switch to stand right beside the monitors so he wouldn't have to be talking from across the room.
"O-oh s-sorry about that," She apologizes "I h-had my headph-phones on so I di-didn't hear y-you." She says as she spins her chair slightly before turning it to face him again. "It's all good Al." He says with a smile.
This reminds him of how they used to be partners in the lab years before but now was not the time to reminisce on times long forgotten.
"hey uhh if it's not too much to ask could ya help me with somethin'?" He asks as Alphys stands up and begins throwing some of the instant noodle cups away that had piled up near the monitor screens.
She turns to him with a smile on her face "Uhh s-sure I'd love to h-help y-you," She says then frowns, "as l-long as it's in r-reason t-that is."
Sans begins pacing when he starts explaining what it is he needs.
"I need you're help with pranking Grillby for next month." He says.
Alphys looked at him and then laughed "O-oh is this to get back a-at him for last y-years prank?"
Sans stopped in his tracks and turned to her "uhhh yeah but how'd you know about that?"
He asks with a confused frown on his face, well as close to a frown as he could get with a permanent smile on his face.
She laughed again and walked over to her laptop, " Y-you don't know about the v-video?" She asks him.
"Uhh no what video." he asks more confused than before.
Alphys opened another window on her laptop and went to work searching through the Undernet on what she was trying to find. After a minute or so she stepped aside to let Sans see.
"T-this video," She giggled, "I don't know how y-you haven't seen it yet."
Sans turned to look at the screen and suddenly his face flushed bright blue. Someone had recorded the prank! It was the entire thing leading up to Sans hacking and coughing as if he were trying to rid himself of non-existent lungs and to the point where Grillby doubled over on the floor in laughter.
Sans turned to Alphys who was laughing when she saw his face turn a bright neon blue.
"S-sorry I couldn't h-help it," she said, "It's j-just I th-thought you knew about the v-video but when y-you saw it y-your expression was pr-priceless!"
She proclaimed as she began to settle down from her giggling fit.
Sans's face returned to its original shade of color as Alphys signed out of her Undernet account to close the video.
Sans faced Alphys after she finished, "well besides that will you still help me?" He asked.
"Oh s-sure I would l-love to help," Alphys exclaimed sitting back in her chair,
"b-but what should w-we do? I mean i-its not like we can p-prank him traditional ways s-since Grillby is m-made of fire."
"Oh don't you worry Al," Sans smiled at her, " I think I know just the thing that'll set my plan a-blaze."
He walked onto the other side of her desk and pointed to her laptop. "May I?" He asked.
"O-of c-course," She replied, "just don't j-judge me f-for m-my wallpaper."
He took her laptop and opened another window very mindful of the Kissy Kissy Mew Mew wallpaper that popped up. He began typing into the search bar exactly what he was thinking of doing. He turned the laptop her way and she peered down at it.
"B-but Sans you can't do the dye p-prank because Grillby i-is made of f-fire." She told him not wanting to bring down his hopes of revenge.
"But Al," Sans began, " We aren't using the conventional means to do this." Alphys gave him a questioning look and Sans could only smile wider when he pulled a package from his pocket.
He handed it off to Alphys and began explaining what it was,
"I found this in the dump a few days ago and it looked like something that could work but I need your help in making it since it doesn't have any left in the package and I also need it in a clear dissolving form so he won't notice it." He explained it was something humans called 'Fire Dye Magic' which was a joke in on itself since it had nothing to do with magic and was just a chemical compound but he wasn't going to bring that up now.
Alphys laughed, "O-oh yeah that should get him b-back f-for sure!" She exclaimed
"B-but it will take at least a c-couple of weeks for the solution to ferment i-into a dissolvible s-substance." She explained.
"That's fine Al as long as it's done by the first I want this to go exactly how I plan."
He began to walk toward the exit but turned around at the last second,
"Oh and please do not tell NOBODY about this." He said before teleporting off.
"B-believe m-me I wouldn't dream o-of doing that." She laughed as she got everything set and began reading of the ingredients on the packaging's back.

~Present Day April Fools~

Sans teleported straight to Alphys's Lab and couldn't help but wince at the startled screech he became faced with. Now theres something you don't want to here minutes after waking up!
"SANS! J-jeez s-stop doing that!" Alphys exclaimed after knocking down her chair from surprise.
"At this r-rate you're going t-to give m-me a s-soul attack!"
Sans backed up a little bit to help pick some of her papers up with blue magic as they had fallen off the desk.
"Sorry Al I didn't mean to SCALE you I keep forgetting." He stated
"But I am in a hurry to finish up stuff before Grillby's opens and that's in a few minutes" He exclaimed or came close enough to it as he could with his lazy lower case voice.
Alphys looked confused but then her face brightened as she remembered "Ooooh o-ok I re-remember now."
She said as she walked over to a table off to the side. Sans bent down and picked up Alphy's chair she had knocked down and followed her over.
Alphys handed Sans a small beaker plugged with a corkscrew at the top.
"Th-this should d-do it," she stammered,
" t-the science b-behind how you m-make this was r-really surprising b-but I'm s-sure you can't stick a-around to discuss t-that when your in a hurry." She smiled.
Sans took the corked beaker and laughed, "thanks for doing this Al I owe you one." He pocketed the beacker
"Hey maybe you and Undyne could come over sometime and watch anime with me and Paps when you have the free time." Sans said as he walked towards the exit. At the mention of Undyne
Alphys's face turned bright red. "Y-yeah th-that w-would be gr-great," She stammered worse than before. Sans smiled.
"Alright then I'll have to have Paps set up a movie night just be prepared to be served his 'special movie night spaghetti'" He winks
"you won't be able to forghetti it afterwards."
Alphys's face turned greenish. "In t-that case I'll be s-sure to bring popcorn," She laughed, " but yeah that s-sounds l-like fun just text me w-whenever."
"Alrighty Al thanks again for the assist." Sans said and with that he teleported away.
"This s-should be f-funny." Alphys smirked sitting in her chair. *HOOOOOOOOOOOOOOOONK*
"Eeeeeeeek!" Alphys yelped jumping out of her chair.
"W-what in Asgore's name!" She looked under her chair to see a bullhorn duck taped to her office chair.
"Saaaaaaans!"
Sans having teleported right outside the door snickers as his first victim gets pranked.
"One down the Underground to go." He smiles and teleports away to Grillby's,

Sans materializes right in front of Grillby's restaurant and walks into the bar with a large smile on his face. Like usual everyone greets him but they get anxious and watch his every move watching as he makes his way to the bar and plops down into his seat. As before a woo-pie cushion goes off and a few people laugh but this doesn't bother him after all they are his woo-pie cushions. Grillby comes over with a thing of ketchup and hands it off to Sans before anything can be said though he starts a conversation with Sans.
"...hey Sans about last year... I hope you don't hold a grudge on me for doing that." His fire flickers and crackles
"...we are good now aren't we?" He asks a frown in his voice as he does so.
Sans's smile becomes wicked while he says, "of 'ourse not Grillbz I would never hold a grudge against you. You are the LIGHT of my life after all."
Grillby frowns down at Sans not totally convinced.
"Ok to show you that I am not SKULLKING, 'bout before I'll buy you a drink." Grillby still looks unconvinced but goes to the back to pick something out.
He turns back around with a bottle of Fairy Floss and sets it down on the bar.
"...alright I believe you but please no pranking my patrons while they're here... I don't want to loose business."
Sans puts on his most wounded face and says "Grillby would I ever do that." To this Grillby just glared at Sans as he turned around to get the shot glasses.
He lays both down and pours the Fairy Floss into each glass. Before either could take a drink someone at a table asks for a refill.
'Perfect timing' Sans thought as Grillby goes to the back and comes out with a pitcher to refill his customer's glass. When Grillby wasn't looking Sans took out the beaker of powder and dumped it into Grillby's shot glass. Immediately the powder dissolved mixing with the Fairy Floss without showing a trace of it left behind.
'Alphys you out did yourself' Sans thinks as he quickly shoves the beaker in his pocket as Grillby makes his way over.
He goes to put the pitcher up and comes back over.
"... where were we?" Grillby asks before he remembers. "...of course we were drinking to say you had no grudge on me."
They both clink there glasses together before both throw their heads back to take the shot. In unison both slam there glasses onto the bar upside down.
Immediately after Sans feels the Fairy Floss's affect before it fades away. Grillby's flames expands before they contract back into there normal position.
"Well there you go convinced?" Sans asks trying to not look too excited that his plan was going... well as planned.
"...I have to say... I thought for sure you'd be trying to get back at me but apparently that's not the case."
Grillby smiles and his flames flicker in warm colors. He begins to turn around but as he does his flames become speckled in numerous colors from pink and purple to green and blue. By now at least half the bar becomes quite as they notice the change in Grillby's appearance. Finally Grillby noticed everyone's silence and he turns to see the entire restaurant staring at him.
"...what?" He asks and after a minute he notices they are staring at him.
He looks down and at this point Sans couldn't hold it back anymore he burst out laughing at the top of his longs and Grillby flickers in shock when he sees he is now speckled in foreign colors. Sans can't stop laughing no matter how many scowls Grillby shoots his way he just laughs even more and by now a few patrons begin laughing too catching onto what had happened. Grillby's flames flicker in embarrassment but with the dyed flames no one could really tell the difference.
"...Sans what on earth did you do?!" Grillby proclaims finally after getting over his shock.
Sans tries to compose himself but when he turns to look at Grillby he begins laughing again this time so bad to the point he falls off of his stool.
Still laughing he says, "Hehe sorry Grillbz you'll have 'ta give me a second hehee I am DYEING here!"
He begins laughing again when realization dawns on Grillby's face.
"...Sans," He begins as Sans pulls himself back onto the stool, "...I really hate you." Sans in turn responded with more laughter
"Pfffffffft hehehe" He wipes a tear from his eye socket, "ok ok I deserved that but you gott'a admit that was PRETTY funny huh?"
At this Grillby can't help it but smirk.
"...alright I can admit it was funny." He then looks down at himself and a frown appears on his face.
"...wait," He asks, "how long will this last?" At this Sans turns his head to face him and smiles.
"Sorry Grillbz but the package said 24 hours." At this answer Grillby groans and face palms his head.
"...well this is going to be a very long night then." He grumbles and with that Sans started laughing once again.