Jeff was thinking, and remembered it was his sister's ninth birthday today. "Well.. She did say she wanted to know what it was like to be a boy... Maybe she'd enjoy it if I temporarily became her penis...? I don't have any other presents, so it could be worth a shot."
He walked over to the door, and was about to open it before stopping himself. "Wait, if I'm going to become her penis, I should let her have the true experience." So, he went back to the Chronivac to change it so that his penis was used instead of her head. "That should be good. Now to find Alice."
He walked out of his room, searching for his sister. He eventually found her laying on her bed, reading a book. He stepped into her room, and closed the door. "Hey, Alice!"
Alice looked up from her book, and simply waved before focusing back on it. He walked over and sat down next to her, tapping her shoulder. "So, I remember you saying you wanted to know what it's like to be a boy." Alice looked up at him, before responding with "Well, y-yeah. Why?" Her face was a bit red, although she was young enough she didn't quite know about that stuff yet.
"Well, I think I found a way for you to temporarily experience part of what it's like. It could be cut off at any time, but it'd let you feel how it is before you would make any permanent choices." Alice was blushing a bit, but still seemed excited. "Really? How could I do it though?"
He moved a bit closer to her waist, before speaking again. "Well, it'll feel weird, but it'll require you taking off your undies." Jeff then pointed towards her skirt, referring to what she's wearing under it. While still blushing, she reached over, and took her underwear off without Jeff able to see what it was. "Alright, what next?"
Jeff moved over to her waist, as Alice put a bookmark into her book to pay attention to what he's doing. "This is the part that's going to feel weird, and you're just going to have to trust me on this, alright?" "Okay!" Jeff was happy she was still too innocent to know what he was doing.
He got around behind her, and positioned himself just past her skirt. "This'll be a bit easier if you roll over, onto your stomach." She does so without complaint, and Jeff angles his head so that he's ready to push right in. "Alright, are you ready?"
Alice quickly responds with an excited "Yes!", and Jeff prepares himself. After a deep breath, he closes his eyes and shoves his head forwards, crawling as far in as possible. Alice yelps in pain as he does so, before the Chronivac kicks in. Jeff is regressed down to nine years old, and shrunken to have his size, allowing him to fit a lot better into the small space. "I think it should be good."
Jeff's words were muffled, Alice feeling a shiver as she heard them inside of her, but didn't know where it came from. Alice flipped back onto her back before sitting up, Jeff's lower half fitting perfectly between her legs. Alice didn't see Jeff anywhere, but lifted her skirt up, pushing her waist out to see what Jeff did. "Huh? What's this thing sticking out of my butt?"
Jeff heard those words and realized he's in the wrong hole!