Out of the corner of his eye, Jack noticed the Superman posted he had hanging up on his wall. It brought back memories of all the comics he had read up until this point. Superheroes lifting cars over their heads, stopping trains, punching through solid steel walls...
... And there was the idea. The one, most important thing to him right now.
He brought his eyes down to the button. He somehow knew that he only had to think what he wanted, but he said it anyways:
"I wish I was the biggest, strongest, most muscular being on the planet."
And with that, he pressed the button.
Instantly, it felt like there was an explosion ripping up from his hand over the rest of his body. Jack cried out, almost dropping the button from his hand instinctively. He felt so much pain... and yet, oddly enough, the pain was almost mixed with a little bit of pleasure. Like there was also a nice heat making its way across his form.
Jack felt his chest starting to strain a bit. At first, he thought it was because his heart was beating so fast. Well, his heart WAS doing that, but when he looked down, he could see that his chest was straining for a completely different reason - he was GROWING! Good, honest to god, straight up growing. It almost looked like he was taking breaths and his pectorals were just... staying there. And the cycle kept repeating itself. Right in front of his eyes, Jack was seeing his chest start to stretch out his Batman t-shirt, partially obscuring his view. Not letting that stop him, he reached down, smiling widely as he felt abdominals pop up on his stomach, forming a solid six-pack... which then split up and grew even further into an eight-pack!
Ignoring the increasing sound of stretching fabric, Jack reached down further and wrapped his hands around his swelling thighs. That task was becoming increasingly difficult as the muscles expanded in diameter - not only did they feel as big as grapefruits, they were getting insanely hard as well. Jack could tell that these muscles weren't just for show.
Jack almost felt like having a heart attack when he realized that the best part was still to come - the biceps. Not wanting to miss the show, he raised both his arms, almost letting out a yell of happiness as even that minor action caused them to gain a couple of inches all around. It felt like he was watching something out of a cartoon - his arm muscles went from the size of a baseball, to the size of large rock... all the way to what almost looked like soccer balls stuffed under his skin! Even better than the sight of it was just the immense feeling of adrenaline and power Jack felt, knowing how strong he probably was.
Almost as quickly as it started, Jack felt the growth fading, eventually dying out completely. He was just left to catch his breath, stand there, and dwell on everything that had just occurred.
Even breathing made him feel like a titan, watching his pecs nearly burst out with every single inhale. And that wasn't even getting to the rest of his body. Getting curious as to how it would feel in motion, Jack took a cautious step forward. It was... an odd sensation. He knew his body was heavier, and he felt it, but at the same time his legs were so strong it felt like he could leap a mile in a single bound. This was unreal.
Jack turned, looking at a full-length mirror on the other side of the room. Seeing his full body just made him feel ten times better. He was AMAZING! It almost looked like some kind of cartoon 0 Jack was unbelievably swollen, but in all the right places. Humongous pecs, abs that looked as hard as steel, legitimately INSANE biceps... it almost took Jack a second to realize his leg growth had made him even taller. He was probably around 6' 6" now, a tall height which just made his muscles look all the more intimidating. In fact, he was surprised his clothes had even survived the growth... though his Batman t-shirt was stretched so thin he could see his skin through it, and his shorts looked like they were painted on his body. Finally, Jack looked up at his own face, surprised to see that it had received minor changes as well. He now had a slightly more square jaw, and even a little bit of stubble on him. It was still obviously him, just a bit more... strong-looking.
Jack just stood there for a good while, admiring his own looks. This was all he had ever wanted! He didn't know how the button had actually worked, but he didn't care. He was huge, super strong, and quite good-looking, if he could say so himself. In fact, now he was thinking about -
Before he could finish that thought, there was a knock at the door.