This was Sans' favorite day, apart from the anniversary of leaving the underground. Sans glanced at his calendar, and widened his ever present grin. April 1st, the day where pranks are played all over the world. Or, at least if Sans is there. The first thing this overweight skeleton did was wrap plastic wrap all over Papyrus' door. He chuckled deeply, and replaced Papyrus' spaghetti noodles with macaroni noodles. He replaced the tomato sauce with ketchup.
Sans went over to Toriel's house, and quietly used a 'shortcut' inside. He filled the inside of Galahad's armor with paper. He put Galahad's sword in a rock. Sans carefully replaced the pictures inside of Frisk's locket with pictures of him. He placed the original pictures on the bedside table. He went downstairs, and filled the front of the refrigerator with ketchup.
Sans went outside, and discreetly popped the tires on Toriel's car. He walked home, and popped the tires on Papyrus' car. He placed a couple banana peels here and there, and placed a tack on Papyrus' favorite chair. He then simply waited for Pap to wake up.
"hey bro!"
"WHAT BROTHE-AAAAH!"
Papyrus fell down the stairs, wrapped up in plastic wrap, with a banana peel askew on his head. Once he finally got himself untangled, Sans offered his hand.
"high five, bro. you got out of the best of my pranks."
"WELL, THANK YOU SAN-OW!"
Papyrus was standing there, nursing his hand, as Sans had a joy buzzer.
"okay, i'll take off the joy buzzer." Sans removed the joy buzzer. "now, high five again, nothing in my hand."
Papyrus slapped Sans' hand.
"PPPPPPPBBBBTTTTT!" The whoopee cushion in Sans' hand deflated, scaring Papyrus to fall backwards.
Sans raised his hand, showing the whoopee cushion. "whoopee cushion in the hand trick. happy April Fools, bro." And Sans walked away, sipping from a ketchup bottle that he didn't have a second ago.