What prompts Scout to think, "Jem is the one getting more like a girl?"It's in the book To Kill a Mockingbird in Chapter 25
At the very beginning of Part Two of Harper Lee's novel, To Kill a Mockingbird, Jem is feeling growing pains and scolds Scout that "It's time you started bein' a girl and acting right!" A tearful Scout responded to Calpurnia that all Jem needed was to get beat up, "and I ain't big enough."
Scout sees Jem's feminine side emerge in Chapter 25 when he takes pity on a roly-poly that she is about to squash.
"Don't do that, Scout. Set him out on the back steps."
"Jem, are you crazy?..."
... Feeling sleepy, I decided to end things. My hand was going down on him when Jem spoke.
Jem was still reeling from the conviction and death of Tom Robinson, who he recognized as a crippled, innocent man who had been wrongfully punished--one of the story's mockingbirds. Jem was likening the roly-poly (a type of caterpillar) to Tom--to a mockingbird--small and weak, "they don't bother you."
"Reckon you're at the stage now where you don't kill flies and mosquitos now, I reckon," I said...
Jem was the one who was getting more like a girl every day, not I.