Final answer:
The thought of potentially breaking an ankle signifies the character's immediate risks, mental anxiety, and the theme of man versus nature.
Step-by-step explanation:
The brief thought of the boy worrying about breaking his ankle while running through the grass can be seen as significant for a few reasons. Firstly, it symbolizes the immediate risks and challenges the character faces in his environment. It elevates the tension and stakes for the character, as even the ground he runs on can't be trusted. Secondly, it addresses the boy's mental state, showcasing his anxiety and the panic that foreshadows potential failure and deterioration in his physical capability. Lastly, this anxiety relates to the overarching theme of man versus nature, where nature's indifference to the boy's plight is highlighted, as he could be incapacitated by the simple act of running through grass.