THE WORKING PRINCIPLE of a railgun goes like this:

Take two rails connected to a common source of electricity and place them parallel to each other. Now take a conducting material, say a coin, and stick it in between the two lines to complete the circuit. From Ampère's circuital law, it follows that a clockwise magnetic force forms around an electric current in the direction of the current. This effect is doubled when two lines of current moving in opposite directions sit antiparallel to each other. In between the two currents exists a doubly strong magnetic force that points upward and perpendicular out of the plane of the currents. As electrons flow through the coin, a Lorentz force pushes the electrons away from the field, and so the coin becomes magnetized. The coin then shuttles down the two rails of current, away from the power source at blinding velocity.

Tōma closed the physics textbook and grabbed his jacket. He was supposed to be at the library in five minutes.

A blast of cold air met his face as he stepped outside and closed the door behind him. The first snow of the season fell on Academy City yesterday and covered it in a thin, white veil. Today, the sidewalks were already being pelting with slush from the asphalt. Tōma followed the trail of footsteps branded in ice, careful not to disturb the fresh white on the sidewalk.

He sighed.

Tōma had locked himself in his room to study Maxwell's equations for four hours straight. At least this time, he understood most of the concepts. Six hours earlier, he got back his thermodynamics exam: 68 compared to a median of 87. Had he only listened to Mikoto, he told himself, then he would have gotten an 80. The three part question on expanding gases cost him 12 points; he confused positive work with negative work.

A car drove by him and dumped black slush by his feet. He glanced at the spot where he and Mikoto kissed for first time. The cold of the rain, the warmth of her lips, her nervous touch. He remembered her fingers weaving through his. Since that night, they kissed a few more times. Mikoto still hadn't divulged to anyone that they were a couple. She told him she was afraid that Kuroko might teleport into his room and kill him in his sleep if she ever found out. And so they only met at the café after Kuroko had gone to bed.

The library appeared before him as he turned the corner to cross the street. Students flocked in and out of the doors to and from dinner. Tōma entered the building and found Mikoto lounging in the fiction section, not quite enjoying a book.

"You're late," she said without looking up.

"Sorry, I was just reading up on something," he said as he found a seat across from her.

"Reading up on what?" she said.

"You remember the first time I met you on the bridge, you asked me if I knew what a railgun was?" he said. Mikoto nodded. Tōma continued, "I just found out today how it actually worked. Apparently, you're supposed to have two parallel currents around the coin, but I've never seen currents like that in all the times you've shot your railgun. How come?"

Mikoto smiled. "Heh! You thought you could stump me with a question like that, did you?"

"Well, no. I was just—"

"Look, the answer's pretty simple," she said. "It's because the currents are so short, that you can't even see them."

Tōma thought about this for a moment. To accelerate a six gram coin to three times the speed of sound over a distance that he couldn't even see? That meant that Mikoto must have been generating an astronomical amount of energy from her hand, something to be expected from the most powerful esper on Earth.

"By the way," she said. "What did you end up getting on your thermodynamics test?"

Tōma fidgeted in his seat. "Can we not talk about that for now?"

"Not good, huh? Well, that's not a big surprise."

"You certainly have a lot of faith in me, Biribiri," he said.

She leaned back and crossed her arms. "Hmph. How do you think you're going to do on this next exam?"

"A lot better than the last one."

"Oh?"

"Yeah, I actually get the concepts this time."

"Glad to hear it," she said as she turned away and pursed her lips.

Mikoto brushed her hair back, and her scent wafted towards Tōma. That scent like a toy motor or bumper cars at the amusement park, it didn't go away no matter how much perfume she put on to hide it. It was ozone, Tōma recognized. An electric spark provides enough energy to combine atmospheric oxygen into ozone. She tried to mask it with perfume, but he could always tell, and she knew that he could tell and that he didn't mind. She liked him even more for that.

"One thing, though, about your railgun," he said. "Isn't the coin powered by magnetism and not by electricity?"

"Technically, yeah."

"So your electricity produces the magnetism, right?"

"Yeah."

"So, shouldn't you also have magnetic powers too?"

Mikoto scrunched her face to one side and shook her head.

"Idiot," she started, "it's not that easy".

"What do you mean?" he said.

"I mean, it's like trying to use chopsticks to hold on to a spoon and then using that spoon to eat," she said. "Sure, I might be able to use the spoon after some practice, but not as well as just using the chopsticks directly".

Tōma nodded and wondered to himself how Mikoto came up with such an apt analogy so quickly.

"So are we actually going to study physics?" she said. "I didn't come here just to entertain you, you know."

Tōma pulled his physics textbook from his backpack and opened to the section on Maxwell's equations. For the next thirty minutes, they cruised through Biot-Savart's law and Gauss's law for magnetism. Somehow, Tōma absorbed these concepts faster than Mikoto did when she first studied them. Almost jealous, Mikoto pounced on his errors with a flurry, and Tōma never made the same mistakes again.

The day was dead by now. Most of the students had left for dinner, and the study space vacated in a hurry. Mikoto looked out the tall window. Outside, two neat rows of dotted light, one white, one red, flowed past each other on the road. They reminded her of the two currents of her railgun. Mikoto thought to herself, if she extended the current for a few more meters, she could accelerate the coin even more and score higher on her next pool measurement test. How come no one ever told her she could do this?

"...Maxwell?" said Tōma. Mikoto only caught the last word.

"Say that again?" she said.

"I said, who exactly was Maxwell?"

"A Scottish physicist who lived in the 19th century," answered Mikoto. "Maxwell was the theorist. Faraday was the experimentalist, but both were great scientists."

"Scientists?" produced a high-pitched voice from behind Tōma. Mikoto locked up quicker than Tōma could turn around.

"I didn't think the apple of the Ace of Tokiwadai's eye had a scientist fetish."

Behind Tōma rolled in a petite girl on a wheelchair. Her rosy pigtails drooped down to her shoulders and surrounded a devilish grin that hung on her face.

"K-K-K-Kuroko!" said Mikoto.

"So tell me, Onee-sama," said Kuroko. "What might you be doing here with this man?"

"We-we were just studying," replied Mikoto.

"Really?" she said. "You're awfully red, Onee-sama." Kuroko rolled up to the table between Tōma and Mikoto. "Do I detect a hint of love in the air?"

"Kuroko, is it?" said Tōma. "Didn't I save you a few months ago?"

"That's right!" she steamed, almost lunging at Tōma. "And that's the only reason you're still conscious!" Kuroko gnashed her teeth and tried to impale him with her stare.

"There's some relationship between you two that I don't know about, isn't that right?" she continued.

"Actually," Tōma said, "we really did just come here to study."

Mikoto kept her head down and arms between her legs to hide her glow. She trusted that Tōma could talk for her.

"Regardless, I came here to retrieve Onee-sama," said Kuroko.

"For what?" asked Tōma.

"If you must know, Tokiwadai Middle School is hosting a Christmas dinner for all espers level 4 or above," she said. "It's in an hour, and attendance is mandatory."

Tōma scratched his head and said, "Well, I guess that's fine then."

"Oh, it better be fine!" said Kuroko. "I'll be out of my wheelchair in a week, so we can duel then for the Ace of Tokiwadai's honor!"

"Duel?" repeated Tōma.

"Kuroko," said Mikoto. "I don't think that's necessa—"

"It is necessary!" said Kuroko. "There's no other way to settle this."

"Yeah, but I'm a level 0," said Tōma. "And you're a level 4 if I'm not mistaken. It wouldn't be fair."

Kuroko scoffed. "You try to fool me with your words, but that won't work," she said. "You are the one, after all, who saved me from Awaki Musujime."

Kuroko clenched her fists and bit her lip. Tōma produced a nervous smile.

She reached over and grabbed Mikoto's arm, still limp between her knees.

"Come on! We're leaving!" said Kuroko as she wheeled for the exit. Tōma stared helplessly as Kuroko dragged Mikoto into the night. From outside, Mikoto shot Tōma a final glance before she disappeared around the corner. Tōma shrugged and closed his textbook. He decided he would call Mikoto tomorrow.

Kuroko refused to let go of Mikoto even as Tōma phased out of view.

"I don't know what you see in that man, Onee-sama," said Kuroko without looking back. "He reminds me of the criminals I catch for Judgment."

"Kuroko, could you let go of my hand?" said Mikoto.

"Only if you promise to stay away from that guy."

"Okay."

Kuroko turned around and studied Mikoto's face for a tell. Unable to find one, she eased her grip on her superior's hand.

"Don't worry. I'll win back your honor, Onee-sama," she Kuroko.

They continued down the sidewalk. The snow began to harden as the December winds continued to beat on the ground. Mikoto emitted a weak electric field to heat up the air around her. That was how she could wear her summer uniform even in the winter.

"Like I said, Kuroko," said Mikoto. "You don't need to do that."

"This is out of your hands now, Onee-sama," said Kuroko. "I've decided already, and when I've decided on something, it's final."

Mikoto halted in her steps. Kuroko turned around. "Onee-sama?"

"I just remembered something," said Mikoto.

She reached into her pocket and retrieved a coin, five grams, 75% copper, 25% nickel. She placed it on her thumb and then flicked it up. Kuroko tracked the coin as it hung in the air.

Mikoto shot out two long rays of electricity around the coin, and a hundred yen launched into the night in a beam of orange energy, vanishing among the stars.

"That was faster than usual," said Kuroko.

"A lot faster," said Mikoto.

Satisfied, Mikoto smiled and walked past Kuroko and began to jog towards Tokiwadai.

"Wait! How'd you do that?" said Kuroko as she wheeled after the railgun.