The surprise ending of Hemingway's story "A Day's Wait" begins when the sick boy asks his father, "About what time do you think I'm going to die?"
The reader has been assuming all along that the nine-year-old boy is just suffering from a case of the flu. The doctor told the father "there was nothing to worry about if the fever did not go above one hundred and four degrees."
The boy they called Schatz had a temperature of one hundred and two, but he was too young to understand there was a big difference between Fahrenheit and Celsius. It would appear that the family had been in Europe and is now in America, where Fahrenheit is standard.
When Schatz asks what time he will likely die, his father begins to understand that the young boy has been lying there all day expecting to die. The reader and the boy's father understand Schatz has been displaying great courage, not unlike a grown man who was terminally ill and expecting to die very soon. The reader experiences many of the feelings the father must have felt at that time. In addition to relief, the father must have admired his son's bravery at the same time he pitied the small boy for the mental anguish he must have endured during that long day in bed.