What is the most grown up thing that Jerry does in "Through The Tunnel"?
After completing his "rite of passage" by swimming through the tunnel victoriously, Jerry has established his maturity first with his daring and diligent practice that has led to his victory. Previously, his "chivalry" to his mother has involved an "unfailing impulse of contrition"as he runs after her to accompany her to the safer beach; however, after being around the daring boys who have disapproved of him, Jerry has become their equal by performing the same feat as they.
When he emerges from the deep waters after successfully passing through the underwater tunnel, Jerry perceives these local boys as they dive and play in the distance. However, he did not want them" because he, too, is now mature. Instead, he returns to his mother and has lunch with her, telling her only that he can remain under water for two or three minutes. Characteristically, his mother is "ready for a battle of wills," but Jerry no longer needs to fight for his independence with her: "It was no longer of the least importance to go to the bay." Now, since he has established himself as a young man with his act of daring, Jerry's second act of maturity is evinced in this thought, and it is his most grown up act of all. For, confident in himself, Jerry is no longer threatened by his mother's attempts to protect him.