AN: I love the Lily/Severus friendship and wanted to write this. It's just a bit of fun and not to be taken siriusly.
It was half nine at night and down in the potions classroom two Fifth Year students were scrubbing the stone walls and floor. One of them was a Slytherin boy while the other was a Gryffindor girl. Lily Evans was loudly voicing her hatred towards those who caused this as she stood on a table to reach some of the stains which splattered higher then she could reach. Severus Snape scrubbed at the floor vowing that if any of those four had a child; he would become a Hogwarts teacher just to make that child's life a nightmare.
Earlier that day
Potions was the class both Lily and Severus were the best at. While the others worked on OWL potions, Professor Slughorn allowed his two best students to attempt NEWT level work. They were dead certs to get an O in their exam this summer. As Lily returned with an extra ingredient, Severus spotted a firework fall into the cauldron.
"Get down!" Severus yelled, taking his best friend's hand they ducked under the desk as the cauldron exploded. The potion splattered the walls, the floor, and the students who didn't duck in time as well as Professor Slughorn. Wiping his face, Slughorn dismissed the class and turned to Lily and Severus who could hear the victorious laughter of those bloody Marauders as they made their way out of the dungeons.
"Maybe we should leave the NEWT potions until next year. After dinner the two of you will clean this dungeon without magic." Slughorn ordered. Lily and Severus sighed and went to dinner. When they had eaten and gotten an hour long rant from Filch, they set to work cleaning the dungeon.
Severus put his brush into the bucket and Lily jumped down from the table. They surveyed the now spotless dungeon and went to put away the cleaning equipment. As they shut the broom cupboard door, Severus turned to Lily.
"We need to get our own back. Those idiots deliberately exploded our cauldron. I'm sick of them acting like a bunch of kids." Severus said.
"You're right. Hexing and playing harmless pranks is one thing, but someone could have been seriously hurt today. For all they knew we could have been making poisons or something equally dangerous. I mean, we knew the potion was safe but those three didn't." Lily replied leaning against the door.
"Those three? Who was missing?" Severus asked.
"Remus, Professor McGonagall kept him after Transfiguration today for his careers advice session. Remus is the only one who has some common sense. Bottom line is that Potter, Black and Pettigrew should be taught once and for all that their arrogance will bite them on the backside." Lily explained.
There were a few moments of silence before Severus had a brilliant idea to really show those three idiots what he and Lily thought of them…
"I think I know how," Severus said and whispered his brilliant plan into Lily's ear. Quickly and quietly they rushed back to the dungeons and began brewing, silently laughing and waiting to see three of the Marauders get what they deserve.
James, Sirius, Remus and Peter entered the Great Hall the next morning. Remus had been told all about the exploding cauldron and wasn't surprised to see a tired looking Lily glare at his three best friends as they sat down beside her. Remus poured himself milk and Sirius leaned over him to get the pumpkin juice jug beside Lily. Despite her tiredness, Lily felt a flutter of excitement as Sirius, James and Peter all poured themselves juice from the jug he had spiked as they entered the Great Hall. Lily glanced over at the Slytherin table and gave Severus a small thumbs-up as the fun began.
Within seconds of the three pranksters drinking heir pumpkin juice, their heads began to swell to an enormous size. The entire Great Hall began laughing and the teachers rushed to help the fifth years. Most of the students were standing on the benches to get a better look while the teachers attempted to stop the swelling and restore calm. Through the chaos, Lily managed to slip away and met Severus in the Entrance Hall.
They looked at each other for a moment before laughing hysterically. Spiking the pumpkin juice with a Swelling Solution had been a stroke of genius. It was their way of showing everyone how big-headed the Marauders were. Severus and Lily managed to control themselves as James, Sirius and Peter were taken to the potions classrooms for the antidote. Before ether of them could get up to go to Charms, Professors Dumbledore and McGonagall stopped them.
"Ms Evans did you see anyone put anything into the boys' pumpkin juice?" Professor McGonagall asked.
"Sorry Professors, I was tired after my detention and staying up to do my homework and I wasn't paying attention in the Great Hall until they started panicking." Lily replied.
"Well hurry along both of you or you'll be late for your classes." Professor McGonagall replied and walked off. Severus and Lily went to climb up the stairs when they heard the headmaster speak.
"I would like to think you will not be making a habit of this and instead concentrate on your studies instead of foolish tricks. I'll expect top marks in you Potions OWL in June for both of you or I may change my mind about punishing you both." Professor Dumbledore said with a smile and walked off.
"He's a bit mad in the head isn't he?" Severus said as they climbed the stairs.
"Maybe, but he didn't punish us although the look on their faces when their heads began to swell would be worth any amount of detention. That was the best idea ever." Lily grinned and hugged Severus tightly as they arrived outside the Charms classroom.
Nearly 50 years later
The portrait of the dead headmasters and headmistresses of Hogwarts were eavesdropping as Professor McGonagall told off a First Year pupil for spiking the drinks of the Slytherin Quidditch team, including her older brother's.
"They kept rubbing in the fact they were the Quidditch champions and were being so big headed. I just wanted them to see just how big their heads had swollen because of it." Lily Luna Potter protested.
"Just because Slytherin won the Quidditch Cup is no reason to spike the team's pumpkin juices with a Swelling Solution." Professor McGonagall replied rubbing her temple. She was getting too old for this.
"They were fine. Al even told me that they all thought it was a good prank." Lily replied.
"That is not the point Ms Potter," Professor McGonagall replied but was interrupted by one of the portraits.
"Now Minerva, there was no lasting damage then the girl shouldn't be punished, despite the fact she's one of Potter's children. What's your name?" The portrait of a black haired, hook nosed man asked Lily who looked stunned at being addressed by a former headmaster.
"Lily Potter, sir." Figures, Snape's portrait thought.
"What year are you?" he asked.
"First year," Lily replied.
"A perfect Swelling Solution at your age? Impressive, even though it's just second year work." Snape replied.
"It's my best class sir. Professor Slughorn says I take after my Granny Lily." Lily replied.
"Indeed you do." Snape replied and gave Lily a small smile which she returned.
"Thank you Severus. despite the skill of your potion making Miss Potter, twenty points from Gryffindor. You may go." Professor McGonagall replied.
"Yes professor," Lily replied and left the office.
Snape thought about the young girl who had just been in the office and remembered her namesake who had pulled the exact same prank a long time ago when life was easier and Lily Evans was his best friend…