How does the man determind that it is colder than fifty degrees below zero in "To Build a Fire"?
The man is a newcomer to the Yukon territory, but he knows that at fifty degrees below zero, one's spit freezes when it hits the ground. He spits, and the spit freezes in midair. This tells him that it is actually considerably colder than fifty degrees below zero, which, London tells us, means little to the man, other than that it is very cold. London juxtaposes the man's response to his situation to that of the dog, which "knows" that it is actually about seventy-five degrees below zero, and furthermore, that "it was no time for travelling." The dog, in short, is more reliably informed by its instinct than the man is by his intellect. It does not take a leap of imagination from this point relatively early in the story to figure out what is going to happen to the man.