Author's Note: What to say? I'm a long-time lurker of fanfiction sites, but I've never posted anything before. So I guess I'm writing a romance about Endgame Hulk. Because why the hell not. Enjoy?

The Formula for Change

One: Kinetic Energy

"Oh, please pick up, please pick up, please pick up," Tegan whispered over and over into her phone as the car pulled out onto Washington Street and proceeded in the direction of Boston's North End.

Beside her in the driver's seat, Benjamin Goodall gave her an exasperated look. "Stop worrying, we're only ten minutes late," he told her.

She waved her hand to shush him. After a moment, she heard a voice in her ear: "Brown Butter Bakery, how may I help you?"

"Yes, hello, this is Tegan Thomas," she said quickly, silently thanking whichever divine being had answered her prayers. "My fiancé and I have an appointment at eleven A.M. for a cake tasting, and we're going to be a little late. We're on our way, though, so please don't give away our time."

"Not a problem, Ms. Thomas. We'll see you when you get here."

"Thank you so much! And sorry!" She jabbed the 'End' button and tossed her phone into her purse, pressing the heels of her palms into her eyes. "Crap on a cracker."

"Would you relax?" said Benjamin, peeved. "They're not going to give our appointment to someone else."

"Why wouldn't they?" Tegan retorted. "We're not exactly reliable customers. We've already cancelled on them twice now. Frankly, I'm shocked that they still want our business."

Ben rolled his eyes. "It's just cake, Tegan. It doesn't have to be anything fancy. I don't know why we can't just get a couple of sheet cakes from the bakery section at Crosby's."

Tegan stared at him blankly, not sure she had heard him correctly.

"What?" he said.

"That is literally what I suggested. You're the one who said sheet cakes were tacky."

His brow wrinkled as he stared out at the road. "I never said that," he replied.

"Those were your exact words!"

Ben shook his head as he turned the car onto State Street. "Don't bite my head off for saying this, Tegan, but I'm kind of worried about you. You seem really stressed, and I think it might be taking a toll on your mental health."

Tegan took a deep breath, trying to remain calm. This was not the time to get into another argument. "I realize I'm stressed," she answered evenly. "I've had a lot on my plate. I just took on a bunch of new clients, my landlord is dragging his ass about my broken oven, and my mom is on my case because we still haven't sent out the invitations."

"Your mom... means well," said Ben slowly. "But it's not her wedding. It's ours."

The words were out of her mouth before she could stop herself: "Is it?"

For a long moment Ben was quiet. "What the hell is that supposed to mean?" he finally asked in a low voice.

Tegan sighed, suddenly tired. "It means I'm frustrated, Ben. I feel like I've been planning a wedding by myself for the past three months. I know you're busy, but so am I. And yet I've found the time to book the venue and the caterer, find the dress and the tux, order the flowers, make the party favors. I even made the decorations for the tables, from crap I found at the craft store. You could have helped at any point, but you haven't lifted a finger."

"I haven't helped because it didn't seem like you wanted my help," Ben fired back angrily. "I have offered so many suggestions, Tegan, and you shot them all down."

"What suggestions have you made? Name one."

"I can't remember any right now! Not when you're putting me on the spot like this!"

Tegan bit her lip hard. Every damn time, she thought to herself. She could never win. "The only thing I've asked you to do is make a list of the people you want to invite to the reception. And you haven't even done that."

"Because I don't care!" he shouted, pounding the steering wheel with his fist.

Tegan stared at him, taken aback. "What do you mean?" she asked, her voice wavering. "What don't you care about?"

"Any of it! The cake, the guests, the frigging party favors! It doesn't matter! None of it matters!"

In the silence that followed, Tegan's vision grew blurry as she stared out the windshield. Her stomach felt like it was twisted into knots, and she was aware that she had stopped breathing. If she had been in a more objective frame of mind, it would have occurred to her that she was getting used to the sensation.

"Do I matter?" she whispered.

Without warning the car was thrown violently sideways as its passenger side was struck by an SUV driven at full speed. The vehicle spun in a circle, bouncing off of two more cars before skidding to a halt in the middle of the intersection.

For what seemed like hours, Tegan couldn't bring herself to move. Her head was pounding, her ears ringing, and her neck and shoulder were screaming in pain. She thought she had gone blind, until she realized her eyes were squeezed shut. With an effort, she opened them.

The air was thick with smoke and some kind of dust. She could hardly see through it. As the ringing in her ears faded, she could hear screeching tires, frenzied honking, people screaming.

She licked her lips and tasted blood. "Ben?" she croaked.

There was no answer. Ignoring the pain in her head and neck, she turned to the driver's seat. It was empty.

FOUR YEARS LATER

Tegan woke in a cold sweat, breathing hard. The alarm on her phone was going off.

With a groan, she reached over and turned it off before dragging herself out of bed. She gave her shoulder an experimental roll and winced. She really needed to schedule another appointment with her physical therapist. Oh, the irony.

After showering and dressing, she shuffled off to the kitchen on autopilot. It took her three attempts to turn on the electric kettle before identifying the problem.

"I already turned it on."

She turned sheepishly to her best friend and roommate, Hannah Greer, who was sitting at the kitchen table with her mug. "Yeah, I... I realize that now," she mumbled.

"Shoulder bothering you again?" Hannah asked.

Tegan nodded. "Slept on it wrong. How'd you know?"

"Because your hair is still wet. You never bother with the hair dryer when your shoulder is bad."

She couldn't argue with that. She poured out the hot water into another mug and made herself some tea. As she slid into another chair at the table, Hannah's son Jamie came in, his shoes untied and his shirt on backwards. "Morning, Tea," he said, reaching for the cereal.

"Hey, kid," she said, smiling into her mug. "Is that a new fashion statement?"

"Huh?" She gestured at his shirt, and he laughed. "Yep, I definitely didn't do this on accident. All the kids in middle school are wearing their shirts like this."

"That's dope."

Jamie threaded his arms through the correct sleeves and twisted his shirt around. "Is your shoulder bad today?"

Tegan sighed and stood up. "I'm going to go dry my hair," she said flatly.

By the time she managed to tame her hair into submission, Hannah was herding Jamie out the door to drop him off at school before going to work. "See you later, Tea-Cake," she called. "Don't make anything for dinner, I'm getting takeout from the Thai place. I've been craving coconut shrimp."

"That sounds amazing. Have a good day."

As if on cue, Tegan felt something bump into her leg. She looked down to find Scully, her cross-eyed ginger tabby, staring up at her expectantly. "Where the hell have you been?" she demanded. "You heard the word 'shrimp', didn't you?"

After feeding the beast, she left the apartment and descended the stairs to the first floor, the cat trailing behind her. Arriving at another door, she unlocked it and stepped through into the dark bookstore. She switched on the lights and crossed the vast space, feeling a calm wash over her as she took in the rows and rows of books, all arranged exactly as she'd placed them. She went to work, waking up the computer at the checkout desk and turning on the espresso machine. Finally she unlocked the front door and flipped the 'Closed' sign to 'Open'.

The day passed slowly, with only a handful of customers coming in, and fewer buying anything. During a lull, Tegan decided to unload the shipment she had received the night before and add the titles to the store's online inventory. As she worked, she listened to the music on her phone, connected to a wireless speaker.

She was about a third of the way through the shipment when the desk phone rang. She paused the music and picked it up. "Good afternoon, Rag and Bottle Books. How may I help you?"

"Uh, hi," said a male voice. "I don't suppose you have a copy of 'The Feynman Lectures on Physics', do you?" He didn't sound hopeful.

Tegan scrunched up her face in thought. "Richard Feynman, right? It's a three-volume set?"

"Yes!" the voice exclaimed, startling her. "Oh, my God, exactly. Do you have it?"

She was busy typing the name into the inventory search field on her computer. "It looks like I have one copy," she said. "But let me check to make sure it's physically here. Can you hold for a minute, please?"

"Absolutely."

"Thanks." She put down the phone and hurried over to the science section. Her eyes scanned the titles until she found what she was looking for. Drawing out the red hardcover set, she returned to her desk and took the phone off 'hold'. "Success," she said. "I found it."

"That's great! Could you hold it for me?"

"Of course, sir. What's your name?"

"Bruce."

She wrote 'Bruce' on a sticky note and affixed it to the book's cover. "All right, Bruce. I have it set aside here for you to pick up. We're open until five-thirty."

"I'll drop by this afternoon. Thank you so much."

"You're very welcome."

She hung up the phone, and looked over at her cat, who was watching her from her window perch. "Well, that was mildly diverting," she told her. "Now back to our regularly scheduled tedium."

"So in our last class, we talked about the law of conservation of energy. Anyone remember what that states?"

A few hands shot upward in the crowded lecture hall of the Harvard Science Center. From his place at the lectern, Bruce Banner picked one at random. "Yes, go ahead."

"That the total energy of an isolated system is always the same."

"Exactly," he said. "Pretty simple, right? In other words, energy cannot be created or destroyed. Only transformed. Now what does that mean in practical terms? Well, let's say I have a ball, and I throw it up in the air." He picked up a baseball from the podium. "I'm going to try to throw it gently, because I don't want to get in trouble."

The class watched as Bruce tossed the baseball in the air and caught it a few times. "While the ball is moving, it has kinetic energy, energy of motion. As it goes up, gravity causes it to decelerate. It goes down, it accelerates. However, in that fraction of a second when it's stationary, and it's going neither up nor down, the ball has no kinetic energy. Right? Now you're thinking, 'Hey, Banner. You just said energy can't be created or destroyed.' So what happened? Where did all that energy go? Well, for one thing, the ball encountered wind resistance on the way up, so as it collided with the particles in the air, it caused heat in the form of friction. So it transferred some of its energy to the particles around it. It also transformed into potential energy, which is the energy stored in an object. As its kinetic energy decreases, its gravitational potential energy increases." He set the ball down.

"Let's look at another example," he went on, leaving the stage and moving into the audience. He stopped beside one of the students in the front row. "Let's say I pick up Mr. Cartwright here. Do you mind?"

With a nervous smile, the student shook his head and started to rise from his desk.

"No, no, don't get up." In a smooth motion, Bruce picked the young man up, desk and all, and held him over his head. The lecture hall erupted in laughter and exclamations of surprise.

"When I pick him up, I'm using the chemical energy, the ATP molecules in my muscles, to lift him," he said, still holding the man in the air. "But once he's up there, what happens to the energy? Some of it's being stored in gravitational potential energy, right? If I dropped him, it would turn back into kinetic energy. I'm not going to drop you," he assured him, earning more laughter from the audience.

"What else? My muscles generated heat, so that's thermal energy. Even moving him and the desk through the air generated heat. So none of this energy is coming out of nowhere, and none of it is going anywhere. It's just being transferred and transformed." Bruce returned the student to the ground. "Thank you, you're a good sport."

He walked back onto the stage. "Now, let's talk about internal energy. Internal energy encompasses all of the energy contained within the particles that make up a system, including potential and kinetic. This is represented, don't ask me why, by the letter U. So how do we measure a change in internal energy? Let's say we have a container filled with an ideal gas, like helium. If its internal energy changes, it's either doing something to change it, or something is being done to it. So here's how we write that."

Moving over to the whiteboard, Bruce picked up a marker in his large, olive green hand and began to write. "The change, which is expressed by the Greek symbol for 'delta'... What the hell is that? That looks like a gumdrop. Stupid giant fingers. Let me do that again." More laughter. "So change in the internal energy, or U, is equal to heat... which for some reason is expressed by the letter Q. Bunch of crackheads came up with these symbols. Change in energy is equal to heat added to the system, minus the work done by the system. Or plus the work done to the system."

He glanced up at the clock on the wall. "And there are other ways to express this, but we're out of time. So we'll get more into that next week. Have a great weekend. Use up some of that potential energy."

The students stood up and filed out of the lecture hall, saying their goodbyes as they left.

"Bye, Professor Banner."

"See you Monday, Professor."

"Good class today, Professor Hulk."

"Very funny," he said wryly.

Shaking his head, he erased his work on the whiteboard and placed his lecture notes in his messenger bag. As he did so, he saw another note he had written to himself earlier in the day: 'Pick up book'. Cursing under his breath, he pulled out his tablet, which was connected to his phone's hotspot, and looked up the address of the bookstore. It was on Newbury Street, on the other side of the Charles River, just west of Trinity Church.

It was a little past four o'clock now. For a normal person, it was about an hour's walk.

He had plenty of time.

It was a lovely afternoon in late September, and as Bruce stepped outside onto Cambridge Street, he was met by the sight of students milling about Harvard Yard, enjoying the sunshine. He spotted a fellow professor with whom he had a passing acquaintance and waved at him, and received a tentative nod in return. He found that there were more than a few faculty members who still seemed intimidated by him, and he couldn't exactly fault them for it, given his history. He was, however, somewhat of a favorite among the students.

He cut across the Yard and made his way onto Massachusetts Avenue, heading east toward the river. On the way, he passed numerous abandoned buildings, their facades covered in ivy. It was still odd to see what was once a bustling city slowly being reclaimed by nature. It was a chilling reminder of how precarious humankind's position in the universe really was.

He passed over Harvard Bridge and found himself in the neighborhood of Back Bay. Here he was stopped several times by people requesting selfies with him. Bruce was happy to oblige them, but there was a part of him that still felt a sense of shock and disbelief that this was a regular part of his life now. If someone had told him ten years ago that he would be famous for something other than causing billions of dollars in property damage, he would have laughed in their face. Or, more likely, punched it.

Finally he arrived at his destination: a three-story brick building that appeared at one time to have been a warehouse. The sign over the double doors announced the establishment:

Rag and Bottle Books
est. 1989
Rare editions and prints

There was a flyer on one of the doors. Bruce fished his glasses out of his pocket and put them on, and the blurry letters resolved themselves. The flyer read:

Hey nerds! Join our book club!
Thursday nights from 6pm to 7:30
Refreshments provided
"There is no surer foundation for a beautiful friendship than a mutual taste in literature." — P.G. Wodehouse

Bruce smiled. Pushing the door open, he ducked inside and looked around. He was pleasantly surprised. The shop was large and airy, but the space was exceptionally well-utilized. The tall shelves were crammed with books, arranged fastidiously according to author and subject. There seemed to be a section for every subject imaginable: philosophy, ornithology, green living, auto repair, arts and crafts. There were also displays meant to draw interest: 'Armchair mysteries', 'Time travel for young readers', 'I can't remember the title, but the cover was yellow'. He snorted as he spotted a collection of books on cocktail mixology with an accompanying sign that read, 'How to deal with your in-laws'.

There was no one at the cash register, so he decided to wander around. Presently, he was greeted by an orange cat with a serious case of strabismus. It seemed to be looking at him and at its own nose at the same time.

Bruce bent down to pet it, and it responded by leaping onto his arm and climbing up his sleeve to rest on his shoulder. "Ahh, Jesus!" he exclaimed in surprise. "Make yourself at home."

Accompanied by his new hitchhiker, he continued toward the back of the shop. As he did so, he could hear someone humming softly. He rounded another shelf, and found a woman standing next to a book cart, an earbud in one of her ears. She appeared to be in her early to mid-thirties, with light brown, freckled skin and honey-colored hair, which was pulled back in a messy ponytail. She was sorting books and putting them away. And she was singing Hall & Oates.

"High and dryyy, out of the rain," she sang, oblivious to Bruce's presence as she peered at the spine of a book before placing it on the shelf. "It's so easy to hurt others when you can't feel pain..."

Bruce watched her in amusement, trying to decide how to attract her attention without startling her. Fortunately, or perhaps unfortunately, he didn't have to wait long.

"You're a rich girl, and you've gone too far, 'cause you know it don't matter any— Whoa, shit!"

The book she was holding flew out of her hands into the air as her gaze abruptly landed on him. Reacting to the sudden outburst, the cat leaped off of his shoulder, causing him to stumble backward, bumping into the shelf behind him. He stood frozen in horror as it toppled over, crashing into the one beside it, until the rest fell like a row of dominos, scattering books everywhere.

Kinetic energy at its finest.

Author's Note: If you need help picturing Tegan, I've modeled her appearance after Rashida Jones. Because she's great. Plus she's admitted in interviews that she has a huge crush on Mark Ruffalo. So it seemed appropriate.