Mary returns to the house late for dinner and asks Martha to tell Colin that she's too busy in the garden to visit. Martha looks afraid, but Mary isn't scared. She runs back out after dinner, and she and Dickon spend the afternoon working. Soot and Captain busy themselves, and the robin continues to build his nest. They rest after a while and Dickon comments that Mary is getting stronger and looking better. Proudly, Mary says that she's getting fatter and her hair is getting thicker. They agree to return the next day and Mary runs back to the house, excited to tell Colin about Dickon's animals.
The fact that the robin is building his nest while all of this is going on suggests that the secret garden itself is continuing to become more of a nest as time goes on. Now, it's preparing to house Colin in addition to Mary and Dickon. The growth that Mary is experiencing points to the ways in which the garden and its offerings are nourishing her, making her feel happier and more fulfilled, and giving her a sense of purpose.
Martha is waiting in Mary's room and says that Colin has been throwing a tantrum all afternoon. Mary isn't used to thinking about other people, and doesn't see why she should pander to Colin when she wants to spend all of her time in the garden. She doesn't know that it's pitiful when people who feel bad make others feel bad—which is exactly what she did in India—and feels annoyed with Colin for doing this. When she gets to Colin's room, she marches up to him and asks him why he stayed in bed all day. He insists he hurts and asks why Mary never came. She explains that she was in the garden with Dickon.
The way that the narrator describes Mary's displeasure with Colin reinforces that the children can act as mirrors for each other to check their behavior. Though Mary doesn't quite realize that she's behaved in a similarly awful way before, she does recognize now that acting like this is mean and rude. Because of her selfishness, she's able to make this clear to Colin.
Colin says that he's going to ban Dickon from the manor if Mary chooses him over Colin, which makes Mary very angry. She threatens to never come again and the two begin arguing about which of them is the most selfish. Mary defends Dickon, whom Colin refers to as a "common cottage boy." Eventually, Mary gains the upper hand and Colin starts to feel sorry for himself and cry. He laments that he's going to die and develop a lump on his back. Mary irritably says that this isn't true, which Colin finds simultaneously upsetting and pleasing. Mary insists that Colin says those things to make people feel bad and acts proud of it. Colin angrily tells Mary to leave, and Mary walks out, just as angry as Colin.
When the two children argue about who's the most selfish between them, it indicates that Colin recognizes that Mary is selfish—something that, in time, will help him regulate his own selfish behavior. Mary's insistence that there's nothing wrong with Colin except for his attitude brings the idea of positive thinking to the forefront, as it suggests that Colin is creating his own illness by dwelling on it. His pleasure at being told he's wrong suggests that he doesn't actually want to feel this way.
In the hallway, Mary finds the nurse laughing into her handkerchief. She explains that having someone else spoiled to stand up to him is the best thing that could've happened to Colin and says that hysterics and his temper are half of his problem. This doesn't cheer Mary up. She returns to her room feeling disappointed that she didn't get to share the wonders of her day with Colin. She vows to not tell him about the secret garden and thinks he deserves to feel miserable.
The nurse's assessment of Colin's predicament reinforces Mary's assessment: the fact that Colin thinks he's sick is what is making him sick. However, her choice to shut him out reinforces that she's still at a beginning stage of her own development, and will continue to develop empathy and caring as she spends more time in the garden.
Mary finds Martha in her room with a box of gifts from Mr. Craven. The box contains games, a writing set, and several beautifully illustrated books about gardens. Mary feels happy and decides to write Mr. Craven a letter of thanks. She thinks that if she and Colin were friends, they'd play the games and look at the books, and he wouldn't be so frightened and sad. The narrator notes that most of Colin's tantrums stem from the fact that he's terrified of developing a hunchback, something that only Mary is aware of. She wonders if he was upset today because he spent all day afraid, and she decides to go see him again.
Just as when Mary thanked Martha for the jump rope, choosing to use her new things first to write a thank-you letter shows that Mary is expanding her understanding of people around her and now accepts the power of acknowledging kindness. When she wonders if Colin's fear is what sets off his tantrums, it also helps her realize that she has the ability to be kind to others—in this case, by visiting Colin again.