The range of activity 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 practice 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 provide examples of both. By their nature, toy problems can be given a concise and exact description. This means that they can easily be 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 try to give the general flavor of their formulations. unified because reasoning and problem solving can involve multiple areas at the same time. A robotic circuit repair system, for example, must think about circuits in terms of electrical connectivity and physical layout, and about timing for both circuit time analysis and labor cost estimation. Phrases describing time must therefore be able to be combined with those describing spatial arrangement, and must work just as well for nanoseconds and minutes as they do for angstroms and meters. After presenting the general ontology, we will apply it to write sentences describing the scope of spending. A brief reverie on the topic of shopping brings to mind a vast range of topics in need of representation: places, movement, physical objects, shapes, sizes, grabbing, releasing, colors, categories of objects, anchovies, quantity of things, nutrition, cooking, non-stick pans, taste, time, money, direct debit cards, arithmetic, economics and so on. The domain is more than adequate for practicing our ontology and leaves ample space for the reader to make their own creative representation of knowledge. 228 Chapter 8. Building a Knowledge Base Our discussion about