The range of task environments that can be characterized by well-defined problems is vast. We can distinguish between so-called toy problems, which are intended to illustrate or exercise various problem-solving methods, and so-called real-world problems, which tend to be more difficult and whose solutions actually interest people . In this section we will give examples of both. By nature, toy issues can be described concisely and accurately. This means that they can be easily used by different researchers to compare the performance of algorithms. Real-world problems, on the other hand, tend not to have a single agreed-upon description, but we will attempt to give the general flavor of their formulations. unified because reasoning and problem solving can involve multiple domains simultaneously. A robotic circuit repair system, for example, must reason about circuits in terms of electrical connectivity and physical layout, as well as time, both for circuit timing analysis and for cost estimation of labor. Sentences describing time must therefore be able to be combined with those describing spatial arrangement, and must work as well for nanoseconds and minutes as for angstroms and meters. After presenting the general ontology, we will apply it to write sentences describing the grocery domain. A brief daydream on the theme of shopping evokes a wide range of topics requiring representation: places, movements, physical objects, shapes, sizes, grasping, releasing, colors, categories of objects, anchovies, quantities of things, nutrition, cooking, non-stick frying pans, taste, time, money, direct debit cards, arithmetic, economics, etc. The domain is more than sufficient to exercise our ontology and leaves room for the reader to represent their own knowledge creatively. 228 Chapter 8. Building a Knowledge Base Our discussion of