Hoping your sister was satisfied with that little ''potion'' which was probably just a soda of some kind, you start thinking about if you want to buy anything yourself, if theres a store that might have something for you, but before you can do that, your sister jumps up and bumbs her head into yours. You fall to the ground, suprised at how badly that hurt, and then realize it must have hurt much worse for your sister.
''Are you okay?'' you say, only to realize, you sound exactly like your sister! You look up, and see yourself being extremely excited, then you look down, and see your sisters pink shirt and skirt on you, the potion must have worked exactly like the man at the store said! Shit, well all you had to do was bump head with your sister again, and then keep her under control for the next 24 hours, shouldn't be too hard, but before you can do anything, she's running away! She probably want's to swap with others, that would be a disaster!
You start running after her, and see her run into a woman on the way, swapping bodies with her, and then she runs away again! In your little 7 year old body you're barely able to keep up and simply have to pass by your old body, you can't afford to lose her!