Andrew was a lean, blond ten-year old and was staying with his cousin for the weekend. The sound of the doorbell had attracted his attention and he came downstairs to see his cousin looking over the box of vials. "Hey Mark, what're those things?" he asked, entering the room.
"No idea," replied Mark, "Just found it on the doorstep with this note."
Andrew quickly read over the note and turned his attention to the vials as well. "Might as well try one I suppose. You don't mind do you?"
"I guess not. They don't look dangerous."
Andrew perused the vials until he found one that looked interesting; one with a sort of flexed arm on it. Upon downing its contents he felt a strange sort of warmth and pleasure as his clothes began to feel tighter and tighter. When it ended he got back to his feet, having unconsciously dropped to his knees, and realized his skinny jeans and t-shirt were now very tight. Looking down he saw his clothes hugging a lean, muscular body. His shirt outlined smooth, well-defined and rock solid pecs and a tight six-pack below it while the sleeves were stretched by toned, muscular arms. His jeans clung tightly to his round, muscular rear and strong, athletic legs. Altogether he looked like a serious gymnast and grinned as he flexed and admired his new body, delighting in the firmness of the muscles and the strength as he tensed them. "Guess they work," Andrew said at last to the awed Mark.
"Seems like it. Guess you'll be getting an A in gym."