"Ah, sorry!" Nayoung hears a familiar voice before her and looks up from her mobile phone. Her heart skips a beat when she sees a girl bow before another, who is also bowing, before the two figures separate paths. Nayoung narrows her eyes and tilts her head, observing the back view of the girl, wrecking her brain for any recollection of who that voice might be. Was she someone important to me? Nayoung asks herself, slightly concerned.

As she gradually disappears among the crowd, Nayoung releases a sigh and withdraws attention to the school map on her mobile phone screen.

1 new message from Kim Sedong in '347': [bet nayoung unnie's still finding her way there] comes a message that flashes at the top of the screen. Nayoung decides to ignore and swipes it away.

1 new message from Yalmimi in '347': [despite it being her second semester, but like the klutz she is? definitely.] Nayoung clicks her tongue when she sees another one. 

"Heol, it's not like I've had any sessions in this side of the college before!" Nayoung huffs and rolls her eyes at the incoming messages. 

After a tiring time manoeuvring through crowds, Nayoung finally finds her destination. 

"Is this LT2?" she mumbles under her breath. 

"Yes, LT2," a warm voice answers from beside her.

Surprised, she looks up with widened eyes and turns to where the voice seems to belong. 

It's that girl in the hallway, Nayoung thinks. 

"Heol, you're in this class too?" Nayoung blurts, before realising her own words. "Ah-" her eyes dart in anxiety, "I mean- Sorry. Thank you," she bows and enters the lecture theatre. She finds that the girl does so after her. 

Nayoung makes her way up the stairs and seats herself randomly in the middle of a row, with just appropriate a vision she should get of the lecturer at the front. When she places her bag down and takes a seat, she finds that the same girl approaches the row she is at.

Nayoung looks up at the girl.

The girl looks up to her. 

Nayoung sees her eyes. They make eye contact. For the second time that morning, her heart skips a beat.

As if she's been given permission, the girl walks over and settles herself down beside Nayoung.

Nayoung watches all these whilst frozen and dumbfounded. Thoughts run through her mind, as Nayoung is sure to have seen her before somewhere. Or even have known her. Why does Nayoung not recall anything? 

"Perhaps," the girl turns to her, slightly hesitantly. 

"Y-Yeah?" Nayoung almost jumps in her seat. Inwardly, she chides herself, Kim Nayoung why are you stuttering?

"Are you Kim Nayoung ssi?" 

Nayoung feels her heart stop. What?

How?

Nayoung takes a closer look at the girl beside her, whose face is now in full view. Her heart skips a beat, yet again.

"Haebinnie?"

・・・

"So you're in this class too," Haebin asks.

"Yeah, even though my major has probably barely any relation to this class," Nayoung answers casually. "What's your major?"

"Physics," Haebin replies with a small smile and nods her head.

"Figures," Nayoung nods her head several times and turns away as she mutters. 

"You?" Nayoung lifts her head up again to the question. 

"Ah, I major in Industrial Engineering," Nayoung says, and reciprocates Haebin's smile with a sheepish one.

"Okay kids, enough chit-chatting, we'll begin the lecture," says the lecturer as he claps his hands to capture the attention of the students present. "I'll be lecturing you guys for this module on Superposition, and you can call me Park ssaem, for those new to me. I hope all of you have already gotten your notes?"

Nayoung breaks her gaze on Haebin and shifts her attention to the speaking teacher. The lecture theatre is silent and all that is heard are the clicking of pens, the flipping of pages and the scratching of chalk on the blackboard. 

"For firsts, we'll learn about the Principle of Superposition. It's boxed up in the second page of your notes. The Principle of Superposition states that the resultant wave displacement is given by the vectorial sum of the individual wave displacement at the point where the waves meet," recites Park ssaem. Nayoung reads as she listens. 

"You'll see that the heading for this first section of the module is Interference. Interference refers to the phenomenon of two or more waves of the same kind meeting at a point in space to produce a resultant wave disturbance given by the superposition of individual waves at that point. Read closely these two definitions and understand the difference between interference and superposition," Nayoung listens and highlights 'Interference' and 'Superposition'. While she does so, her arm brushes against Haebin's. 

Nayoung freezes at the sudden contact, and watches as goosebumps appear on her arm. 

"Take a look at the first type of interference, known as constructive interference," Park ssaem continues and draws a diagram with two wave motions on the chalkboard, with arrows indicating the directions of each wave motion. "When two waves of equal amplitude approach each other and meet, they form a resultant wave with double the amplitude. We know this from the Principle of Superposition. So you see, they'll create one big one-" Park ssaem then draws another similar diagram, but with a larger wave motion instead, "with double the amplitude."

"But as you can see, the waves then move on after this as they were before, with the same amplitude and wavelength- everything, as they had before," he says as he draws another of the first diagram, but with arrows of opposite direction. 

"I actually see something beautiful behind this," Park ssaem says next and several students laugh. 

Professors see everything beautiful in their major, Nayoung thinks to herself, but it's not like I don't find Industrial Engineering beautiful in any way, she rebuts herself after.

"It's beautiful, but it's sad," Park ssaem's following words make the lecture theatre fall silent as once again all ears are to him. "You know, the beauty in this, lies in that the two waves move on as if they have never met at all. They meet once, create something spectacular, but they part and move on after that without turning back, without changing in any way, just as before. I hope your lives aren't like that. It'll be very sad, if your lives are like that."

Nayoung hears a bell ring at the back of her head. Suddenly, a memory of her last meeting with Haebin flashes before her.

author's notes: when my tutor said that, i knew i had to write smth about it. even though i had no plot in mind. anyway here's a diagram for you guys because it's hard to describe for me and hard to visualize based on my minimal description for you guys... lol superposition is a hard topic that requires visuals... okay all physics concepts requires visuals lol. why am i studying physics