Stanley glances at his canteen, uncertain if he can take a drink of water. Mr. Sir has continued to dump Stanley's water on the ground, though Mr. Pendanski has been making a point to give Stanley extra. It also helps to have Zero digging, but the other boys don't like it. They mock Stanley and insist that he's a master, while Zero, who is black, is his slave. The mocking intensifies when he insists he needs to save his strength to teach Zero to read. Though this is true, Stanley also knows that Zero is a fast learner. He sometimes hopes the Warden is watching them so she'll know that Zero isn't stupid.
Stanley was once relieved that there were no "racial issues," but the way the other boys specifically conceptualize Stanley and Zero's agreement shows that, clearly, he was wrong. Again, this implies that Stanley may have more power than he thought he did based solely on his skin color. The fact that the other boys don't like this agreement again shows that they're far more interested in upholding a system in which everyone has to be miserable.
Stanley sees the water truck approaching. Mr. Sir is driving, so Stanley doesn't finish his canteen. When he hands it to Mr. Sir, Mr. Sir fills it and then takes it to the cab of the truck. After a few minutes, he returns and hands Stanley his full canteen. Stanley is afraid to drink, but takes it back to his hole. Finally, he dumps the water out.
Just as with the Warden's possible surveillance of the boys, when Mr. Sir possibly puts something in Stanley's water, it means that Stanley has to live in a state of constant fear.
Once Zero learns the alphabet, Stanley teaches him to write his name. Zero smiles hugely and seems very proud as he writes it over and over again. Stanley is sad to think that even those hundred zeros still equal nothing. As they head for dinner later, Zero explains his real name is actually Hector Zeroni.