This is the context from "Sherlock season 1 episode 1"
Sherlock(to Watson): "Potential flatmates should know the worst about each other "
Watson(to Mike): "do you told him(him = Sherlock)?"
Mike(to Watson): "Not a word."
Watson: "Then who said anything about flatmates"
Sherlock(to Watson): "I did. Told Mike this morning that I must be a difficult man to find a flatmate for. Now here he is just after lunch with an old friend just home from military service in Afghanistan. Wasn't a difficult leap."
Watson(to Sherlock): "How did you know about Afghanistan?"
It wasn't difficult to figure out. It was obvious. (To make the leap from the facts to the conclusion)
This post is more something for the "Answers" section, and not "Discussions" :-)