What is the unspoken challenge between Stanley and Zero?

In Louis Sachar's Holes, Stanley Yelnats eventually escapes Camp Green Lake shortly after his closest friend Zero goes missing. Eventually, the two boys reunite near a mountain and begin their quest for survival together.
The unspoken challenge between the boys is simply to see who admits that they're thirsty first. The boys eventually run out of water and food (except for the onions, which, along with a supply of water, they'll discover later), and as a consequence find themselves fighting for their lives.
The two boys turn their fight for survival into a sort of game, likely to lessen the severity of the situation. The boys are on the cusp of dangerous dehydration, and by making a challenge to see who will admit that they are thirsty first, the boys are able to keep a strong-willed dedication to survive.