Jeff sat there in front of his computer staring at the screen, wondering what he wanted to add. There were a few options such as whether or not the transformation would be pleasurable, how long it would last, etc. After a lengthy amount of time, he finished the form, and it looked something like this.
===============ADD/CHANGE DNA===================
SUBJECT NAME: Jeff
Age: 16
Height: 5'4"
Gender: Male
DNA being added: 50% Snake
Resulting DNA ratio: 50% Human, 50% Snake
Alter Reality: Yes
Pleasurable: Yes
Age Change: [none]
Duration of Transformation: [1] YEAR, [0] DAYS, [0] HOURS, [0] SECONDS
Aware: Yes
Jeff reviewed his entries before pressing the submit button. He waited to feel something happening. For a moment, he stood there. He figured the program wasn't functioning properly, and almost threw it out, but decided to keep it for whatever reason. It wasn't for a few minutes of lying around before he felt something begin to change. It began with his legs. They began to ache and throb a bit. They wanted to touch each other, as odd as that feeling was. He felt like he would die if he didn't comply with his feelings. He threw of his pants to take a look at the changes that had begun. He also took off his shirt, too, just in case anything happened on his torso. He noticed his penis becoming monstrously hard as the transformation turned got him arroused. As soon as he could, he thrust his legs together, obeying is sudden primal urge. As he did this, his legs began to become tighter and tighter until he couldn't individually move each one. finally, they began to fuse. Both legs became one wiggling appendage and his feet disappeared. In their place was a small rattle. His pubic hairs began to disappear, and the erection he was enjoying up to that point also disappeared along with the rest of genitals. All the hair on his lower body was now gone, and his skin shone, now extremely smooth.
There was an itch as dark green scales began to spread from the base of his rattle, which he could now move freely. He rattled with pleasure as the scales continued to spread up his new appendage. As they reached his waist, they stopped, and his bones began to shift. He now recognized his lower body as a snake tail. Specifically that of a rattle snake. As his muscles, organs, and bones shifted, he felt his spine extend all the way to the end of his tail. It kept going. His tail began to get longer and longer before it was almost 15' long! It became stronger and rounder. He found that he was able to support himself on his tail instead of using legs. The bottom of his tail was a pale tan color, and, with the help of some new found muscles in it, was able to slither about the room. It took some getting used to, having a long snake tail instead of legs, but, he eventually figured it out. He got comfortable on a pile of his own tail, which he had coiled up, and felt the transformation add a few final touches.
He felt his mouth change a bit as his canines grew a bit into somewhat longer, sharp, fangs. They weren't too long, but they reached outside of his mouth. They rested on his bottom lip on both sides of his mouth. He also felt his tounge get longer and, at the end, split into two smaller bits. It was easy to move, and extremely sensitive to touch and taste.
Jeff felt the transformation come to a close, and he looked around. He noticed a few things that he hadn't noticed changing during his transformation. His carpet floor had become smooth stone. His bed was set into the floor and much larger than previously, presumably to accomodate for his huge tail. All of his chairs looked larger, and wherever he looked, he found no pants. His rattle shook happily as he remembered the reality-changing feature. Everyone who ever knew him would think this was normal. History was rewritten to fit his new form, and nobody would know. Nobody but Jeff.
He heard his mother call from the kitchen.
"Come get some supper!" She called.
"I'll be there in a sssssecond!" Jeff replied.
Jeff stopped, he didn't realize that his new tounge gave him such a strong speech issue! He simply slithered over to the kitchen and ate.