There are only three basic constructs in Prolog: facts, rules, and queries. A collection of facts and rules is called a knowledge base (or a database) and Prolog programming is all about writing knowledge bases. That is, Prolog programs simply are knowledge bases, collections of facts and rules which describe some collection of relationships that we find interesting. So how do we use a Prolog program? By posing queries. That is, by asking questions about the information stored in the knowledge base. Now this probably sounds rather strange. It's certainly not obvious that it has much to do with programming at all -- after all, isn't programming all about telling the computer what to do? But as we shall see, the Prolog way of programming makes a lot of sense, at least for certain kinds of applications (computational linguistics being one of the most important examples). But instead of saying more about Prolog in general terms, let's jump right in and start writing some simple knowledge bases; this is not just the best way of learning Prolog, it's the only way ...