A young scientist in the Seattle area designs an experiment to prove she's just as mature as her friends.Kylie Stanton, who's Black, has always been different from her best friends as a budding scientist, a karate brown belt-and an unintentional crybaby. Black and Filipino Naomi, Indian American Nikila, and white Mara never used to mind Kylie's sensitive nature, but everything's changing as they prepare for middle school. Kylie worries they'll decide she's "too babyish to hang out with." Desperate to keep her friends, Kylie designs an experiment for the summer before seventh grade to prove she's mature by not crying and doing everything her friends do. It seems like a foolproof plan, until she finds herself agreeing to things she doesn't want to do and giving up things she does. She's further confused by Naomi, who's been lying, keeping secrets, and being uncharacteristically mean and moody. Is that what it means to be mature? Is that really who she wants to be? Kylie is an endearing, relatable protagonist readers will root for. She wrestles with moral conundrums and situations that cause confusing emotions in ways that are never didactic; this balance keeps readers interested while encouraging them to think independently. Kylie's incremental growth is realistic and well developed, leading to a satisfying conclusion. The strong representation of a loving Black family and the accessible demonstration of the scientific method in action are added bonuses.Heartwarming and empowering. (Fiction. 8-12)