The next morning, Mr. Sir assigns the boys to a new section of the lake to dig their own holes. Stanley is relieved, though his head is swollen and painful where Zigzag's shovel hit him and it makes digging difficult. Otherwise, Stanley isn't sore—the digging has made him strong, and calluses cover his hands. He's still slow, but not that much slower than Magnet.
Note that Stanley is growing strong as he digs just as Elya Yelnats grew strong as he carried the growing piglet up the hill. This suggests that Stanley is unwittingly being prepared for a task of some sort, just like his great-great grandfather.
After Stanley showers, he stays in the tent to write to his parents. As he begins a letter telling them about swimming and rock climbing, Zero walks in and stands behind Stanley. When Stanley asks him not to read his letters, Zero admits he doesn't know how to read and asks Stanley to teach him. Stanley laughs in surprise. He tells Zero he doesn't know how to teach and thinks he doesn't have the energy to teach Zero anyway. Zero insists he only needs to learn to read, as he has nobody to write to. Again, Stanley refuses. The narrator notes that Stanley's heart has hardened over the last few weeks.
Zero's illiteracy is possibly one reason why Mr. Pendanski and the rest of Group D pick on him so much; he doesn't have the power afforded to someone via education to stand up for himself. When the narrator notes that Stanley's heart is hardening, it shows the consequences of existing in such a cruel environment long-term: it's now unthinkable for Stanley to consider anyone but himself.