The Fourteenth Goldfish by Jennifer L. Holm is about a girl whose grandfather, lodged in a teenage boy's body because the old guy thought making himself young again was a good idea, leads her to an interest in science. At least, that was the big attraction for me. Yes, main character, Ellie, is also growing apart from her best friend, a classic middle grade situation. But I got really excited when the science aspect came in.
A particularly entertaining part of the book involves the fact that gramps has turned back time for his body, but not his mind. He's still, in reality, an older scientist, who has now been thrown into a teenager's world.
I thought Ellie's epiphany toward the end was a little abrupt, but, nonetheless, this is a neat book, both entertaining and with some different content.
And a sequel came out earlier this year.