A Bank Street Books Best Children's Book of 2018!

Clover and Danny are the kind of best friends who make each other even better. They're so important to each other that Clover believes they're symbiotic: her favorite science word, which describes two beings who can't function without the other.

But when Danny comes down with a mysterious illness that won't go away, the doctors can't figure out what's wrong with him. So Clover decides to take matters into her own hands by making lists--list of Danny's symptoms, his good days, his bad days.

As the evidence piles up, only one thing becomes clear: Danny is only better when Clover is around.

Suddenly it feels like time is running out for Clover and Danny to do everything they've planned together--to finally see snow, to go on a trip with the suitcase they picked out together. Will science be able to save Danny, or is this the one time when magic can overcome the unthinkable?