Answer: rap, flash
Carl and Russell start to walk when they first get to South America (Paradise Falls). And Carl, sterotyping Russell as a crazy kid, says this to prevent him from doing things that Carl thinks he might do. Like all elderly people, Carl assumes that Russell is an uncivilized youth and that he will be an annoying bother. Right after that, Carl looks back at the house and says, "Don't worry, Ellie. We'll get there."