In chapters 6-8 of "H2O" by Virginia Bergin, Ruby's stepdad tells her to remember the rule of three. The rule of three refers to the fact that humans cannot survive for more than three minutes without air, three days without water, or three weeks without food.
This rule is the most important because it highlights the basic needs that humans have in order to survive. Without air, water, or food, our bodies cannot function properly, and we will eventually die. By reminding Ruby of this rule, her stepdad is emphasizing the importance of staying alive and being prepared for emergencies.
In the context of the novel, the rule of three is particularly important because it is set in a world where water has become contaminated and dangerous to consume. Ruby's stepdad is trying to prepare her for the possibility of having to survive without access to clean water, and he wants her to understand just how essential water is to our survival.
Overall, the rule of three is a crucial concept in the novel, and it serves as a reminder of the fragility of human life and the importance of being prepared for unexpected circumstances.