In the opening paragraphs of Boy, Dahl creates a gentle, lighthearted, personal tone. He does this in part through incidental, informal phrases like "believe it or not" and "this grandfather of mine." Phrases like these create a comfortable, friendly relationship between the narrator and the reader, and this relationship is in turn the foundation for the lighthearted tone of the language.
In the second paragraph, the narrator tells us that his great-grandfather, if alive at the time of writing, "would have been one hundred and sixty-four years old" and that his father would have been "one hundred and twenty-one." These details seem rather inconsequential and incidental, but the narrator shares them with us with the same kind of pride with which a child might boast that he or she is eight and a half years old. These inconsequential details thus suggest a rather childlike, endearing narrative voice and thus contribute to the lighthearted mood of the text.
The lighthearted mood changes to a much more serious mood when the narrator describes his father's fall from the roof of the family home. When the father fell, "two men were called off the street" to help pull the father's broken bones back into their proper places. As the narrator recalls the incident, he speculates that "the pain must have been excruciating." Indeed, he remembers his father screaming, and he remembers too that "a splinter of bone was sticking out through the skin of the forearm." Theses are quite graphic, gruesome details, and with these details the narrator changes the mood of the text from lighthearted to serious.