Having tested it and found you can indeed change people's (or at least your) body parts, you decide it'll be interesting to find out if you really can affect reality with them. You soon think of a wish to test it- what if you were born a girl and everyone thought of you as one?
You focus hard on that idea. I have a female body, you think. I have always been a girl, and everyone will remember me that way. Sure enough, reality seems to contort in response to your wish- in your mind's eye you see the sperm that created you change from having a Y chromosome to an X while it fertilizes the ovum. Instead of having been male since the moment you came into being, you've been female.
A tingling sensation fills your body as it shifts sex. Your figure feminizes and you slim down, curves forming and muscles turning more lithe and petite. Your facial features soften and shift from boyish to girlish, your hair lengthens and your Adam's apple shrinks away to give you a feminine voice. All over you your body hair vanishes and your skin turns smoother. Your nipples expand and you form breasts, your flat chest sprouting large soft mounds of flesh. By now your penis is the only masculine part of your body left, and it too starts to shrink away, growing more sensitive and small by the second. Your balls tuck inwards and start to reshape into ovaries and your scrotum shrivels into your crotch until it forms a flat mound. An indescribable surge of pleasure hits you as the changes finish- what had been your penis finishes its shrinkage and forms your clitoris, and in the centre of your mound an opening forms, giving you a vagina.
When you look in the mirror, you're surprised by who you see, but she also feels familiar. The change in reality hasn't erased your memories of the boy you were, but you also remember being this girl like you'd lived your life as her. You hear your mum call out a girl's name, and recognize that it's yours- you call back to tell her you'll be down in a bit, and go back to looking at yourself, wondering what to do next.