Jack was out on his own, exploring the forest and enjoying the sights and sounds of the place all the while engaging in his typical exercises. He was athletic he knew, but he always yearned to grow bigger and stronger. So he was always up for more exercise, especially when that exercise involved the woods or outland areas. And it was during this exploration that he saw it.
A majestic, albine looking stag, standing proud in the middle of a forest clearing, massive horns upon its head. Jack had never seen anything quite like it during his time out in the woods, and his eyes scanned the brilliant creature over and over. And in that moment the Cabin, its magic following not quite so far behind, knew what it would do to young Jack.
During his silent reverie, Jack suddenly felt a knot in his gut and he grunted. The sudden noise made the stag turn its head and it quickly darted away. Jack sighed, "...Aww, dammit!" He tried to stretch and get whatever that was fixed. He bet it was a strained muscle in his abdominals. He HAD been doing a lot of abs work lately in the name of trying to get a sweet six pack. Not like anyone else in that house seemed to care about appearances.
Jack started to make his way back toward the house, but stopped and clutched a tree tight, the knot in his stomach progressing to a full blown ache and correspondingly seeming to spread into a full on headache. Jack had dealt with SOME issues like this while helping out at school, so he knew well enough to take a seat and wait for it to pass. He groaned - this was really the worst time for this to be happening. Right when he could be out working out or trying to meet up with cute local girls...
Jack rubbed his head, trying to ease out the headache, the young teen wincing and---then it was gone. The ache? Gone. The headache? Also vanished. He looked around and stood back up, "So fucking weird..." He murmured to himself. He rubbed his head and felt two odd point like areas on his forehead. He shrugged and decided to get a better look indoors.
What did grab his attention much more thoroughly was the changes that had happened while he'd sat there. His body was absolutely taut now with muscle, striations Jack didn't even know could exist showing themselves. He shivered a little, but also noted that he was hairier now. His abs had a decent covering going up to his chest, and he rubbed his arms and found the hair there was likewise thicker.
"Huh..." He wondered aloud, "I wanted to grow up faster...just didn't think it'd happen so soon. Wow this is awesome!"
Jack headed home, marching indoors right past a reading and soon gawking Will, who seemed to evaluate Jack as if something was seriously off. Jack for his part however wanted to get indoors and figure out what to do next...