Previous Chapters:
Chapter 0, Chapter 1, Chapter 2, Chapter 3
“And if I only set my own body as frame of reference then … no, wait, that would exclude my clothes. How on earth am I supposed to define my clothes?” Finn let out a scream of frustration. For hours now he’d been sitting in his room, trying to figure out foolproof formulas. The bullet wound in his leg still hurt, but he was too scared he’d damage himself further if he just used a formula to fix his wound. 
It turned out that things weren’t as simple as he had previously assumed. Influencing the simulation wasn’t that hard once he had figured out how it worked, but the program needed very precise input, or things turned out a lot different than they were supposed to. The situation reminded Finn of genies that grant wishes, just to twist them in a way to make the one wishing for it as unhappy as possible.
On a logical level, Finn knew that the simulation wasn’t actively working against him. But it sure felt that way. Then again, how could he know it hadn’t been designed to do just that?
Finn took a deep breath and forcefully exhaled through his mouth. Maybe it was time to take a break and test some of the smaller formulas he had worked on. Time travel was slightly more complex after all, he didn’t have to solve it first.
The mathematician stood up from his desk, took a quick shower, and left the house. It was late afternoon, but the sun was still shining warmly on his face. “Time to produce some vitamin D”, he thought.
About ten minutes by foot away from his home, there was a small park with plenty of trees and barely any people. The perfect place to try out things without nosy spectators.
“Activate silence bubble, twenty meters”, Finn said, and the voice activation of his bracelet reacted immediately. Everything became quiet. The birds, the distant voices of people, the soft howling of the wind, it all vanished.
To verify the effect, Finn tried to say something. And while he felt his vocal cords vibrate, not a single sound was produced. He tried to scream, but still nothing.
Satisfied with the first result, Finn began walking away in a straight line, ensuring his steps, which he was counting out loud, were exactly one meter long. He had measured it at home.
“Twenty-one.” Outside the radius of 20 meters, noises rushed back immediately, just as he had wanted. A satisfied grin spread across his face. His calculations had been correct. A bubble that silenced every sound would come in handy, and there was a lot less room for horrible failure compared to his time travel attempts.
“Deactivate silence bubble.” He made a step back. “Test.” The bubble was gone. Finn wondered if he should implement a numbering system for his bubbles, in case he needed to create more than one. But what if he got them mixed up?
“Why does this have to be so difficult.” Finn sighed. Another thing he’d have to think about back at home, eventually at least. But now, it was testing time.
“Activate invisibility.”
The world went dark, and Finn let out a surprised squeak. He couldn’t see anything! Had the world turned invisible? That didn’t make any sense, he had clearly restricted the invisibility to himself”
“Deactivate invisibility! Deactivate!”
His vision immediately returned, and slowly, his breathing normalized. What had just happened? The formula was supposed to make light bend around his body, instead of being reflected, effectively turning him invisible to any observer. It had seemed like the simplest method. Why had it failed?
Why had it turned him blind?
Suddenly, it dawned on him.
“I’m an idiot”, he said. “A fucking dumbass, that’s what I am. Unbelievable. My eyes need light to see! I bent the light away from my eyes!” Foggy memories of biology class entered his head and made him cringe. Of course, he had gone blind the moment he vanished for everyone else. How could he not have thought of this? He was lucky that he had started testing his formulas before using them in a dangerous situation …
“Woah, how did you do that?”
Finn froze. The voice had come from behind him. Slowly, he turned around.
A child, 12 at most. A girl, holding a melting ice cream cone in her hand. Chocolate flavor, he assumed. And she had obviously seen him turn invisible just now.
“What do you mean?” He tried to feign ignorance. “I didn’t do anything. Shouldn’t you be with your parents?”
“I’m not five, I don’t need to be constantly supervised. And you did something, alright. You turned invisible. And then you screamed, while invisible. And then you turned visible again and started cursing. My mom says it’s not okay to say the F-word, but she always calls my big brother fucking useless, so I guess that’s just a rule that applies to me and nobody else. Or maybe it’s just a rule for kids? Can adults say fuck? At what age are you allowed to say fuck? How did you turn invisible?”
A drop of ice cream ran over her hand, and she licked it off without hesitation, keeping her eyes locked on Finn. He started to sweat profusely. Lying had never been his strength.
“You must have imagined this”, he tried again, but he knew the girl didn’t believe him. She had seen everything.
“I did not”, she said, confirming his assumptions. “I know what I’ve seen. Is lying something that’s also only okay if you’re an adult?”
How could he get rid of this kid?
“Are you a wizard?” She asked. “Or a magician? There’s a difference, but I’ve forgotten what it is. I think wizards are more serious or something. And magicians are cool. I’ve seen Michael the Magnificent with my mom last summer, that was awesome. Do you know Michael? What’s your name?”
The girl jumped from one question to the next, as if her brain was unable to focus on a single thought for very long. But she had just given Finn a way out.
“Yes, yes, I am a magician. My name is, uh, the Amazing Finbar.” Finn clenched his teeth, embarrassed that he couldn’t come up with something better.
“That’s a shit name”, the girl said. “I think I’ll just call you Mr. A. That’s not that bad. Is that okay? Mr. A the magician. Yeah, I like that. I’m Sophie.”
“Nice to meet you, Sophie. But I have to leave now.”
“Are you going on an adventure?” Sophie asked, finishing the last bit of her ice cream. “I want to come with you!”
“No, that’s not possible, sorry.”
Before Finn could even blink, the girl had whipped out a phone and taken a picture of him.
“If you don’t take me on an adventure of you, I’m going to post this picture online, telling everyone your secret identity.”
“Are … are you blackmailing me?”
“Yeah. I guess I am. So, when’s our adventure?”
Finn’s head was spinning. What was he supposed to do? There was no formula for a situation like this. How had he been supposed to anticipate something like what just happened?
“I … meet me here in exactly a week, same time”, he finally said, rubbing his eyes.
“Promised?”
“Yes, yes. Now go.”
“Remember, I have your picture! Bye, Mr. A!”
Finn decided that he really needed a drink.
Winner of the “name our hero” contest is 
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