It was while he was exploring a more forested area around the cabin that the magic of the cabin began to work in earnest upon young Jack. It read his mind and desires and saw his want for size, strength and freedom from the rules being imposed on him. So it began to slowly work to place its energies inside of him, a process which took some time. The decision was made, Jack would in due time be joining the wildlife in the area as a wild grizzly bear, one of the largest and most fearsome of its kind.
Jack felt a weird sensation as energy seemed to pour either into him or right through him - it was like what guys described in those weird ghost adventure stories that Will liked so much. He shivered in place and rubbed his arms, waiting for the distinctly uncomfortable sensation to end. And end it did...just in time for Jack to hear something off in the distance. A distinctive roaring noise - easily identified especially by an outdoors familiar boy like Jack as the roar of a grizzly bear.
He gulped and beat feet back down toward the area closer to the cabin - he wanted some adventure and exercise, but he was not up for an encounter with one of the deadliest animals in any given forest. Once he was away from there the noise seemed to get louder for some reason and then fade away. Jack wondered what could have caused that, but then decided he didn't have to worry that much about it any more.
Instead of hiking around the forest area, Jack commenced doing his exercises one after the other around the outside of the cabin, which was where he noticed the first of many changes;
He usually only got to fifty push ups before his juvenile body betrayed him and he lost the ability to go further, but today for some reason he was feeling much better. He chuckled with glee and kept going, "Haha...fifty one...fifty two...fifty three...fifty four..." Jack kept on going, he had no idea where this strength came from and he was thankful to have it. Springing back to his feet, Jack thumped his chest with one hand...
...And thus noticed his muscles. Yeah, it definitely seemed to the young teen that his chest, his shoulders and his arms were distinctly more muscular than usual. He needed to get a better look. Jack sidled up to the window - which had a decent enough reflective surface - and peeled off his tank top. True to form, his body had...changed. His pecs looked hard as rock on him, his shoulders a bit broader than usual, and his arms at a flex displayed more tone and functional strength than ever before.
"I'm...I am SWOLE!" Jack grinned and pumped his fist into the air, "Oh yeah!" He tossed his tank top on to a clean cut old tree stump and continued with his exercises, feeling more at ease doing them now. Man just wait until Will sees this, Jack thought, see HIM try and tell Jack now not to curse or do what he wants.
And Jack was further delighted when he saw something else after his latest exercise, an indicator of masculine development that he felt he was sorely lacking - a trail of dark brown hairs trailing up his stomach and almost reaching his chest. He grinned again at this and enjoyed what had happened to him so far. Man, was just staying out here making him grow up, become bigger, more muscular and now hairy?
Jack eventually decided to go back inside and swaggered inside past Will - who was still reading his book - and Will basically gawked when he saw what became of Jack; "What in the world happened to you!?"
"I got fucking jacked, that's what happened to me." Jack gloated.
"...Jack, cmon...no cursing remem---"
Jack grabbed on to the front of Will's book suddenly, and the older boy looked at Jack's face, realizing the paradigm had shifted quite a bit in just about an hour. Where was Mom and Dad when they were needed? Jack whispered, "I think I'll fucking curse whenever I fucking want, fucker."
And with that, Jack headed up toward the shower room. He reeked, and he felt he needed to get in a shower as usual after a workout.