How does Expert system works?
System uses a rule base and an inference engine together to simulate human reasoning whenever analysing a problem and arriving at a conclusion. To simulate human processes it's essential to have a vast amount of information stored in the knowledge base and reasoning set out as a set of rules in rule base.
In the above illustration, response Why? Came up as the user wanted to know Expert System's thought processes. System would give the possible response "I am trying to find the make of car you require. Up to now, coupes aren't likely. It's possible that saloon cars are a likely option. I know that you like high performance that gives me strong evidence that car must include a manual gearbox".