To match ideas in our heads, it is inefficient to search through all the ideas that we have whenever we get a new idea. Instead, only a few memorable ideas, or general categories of ideas, are near the top levels of our mind at any given time. When a new idea A reminds us of another idea C that isn’t particularly memorable, then probably A reminded us of big idea or category B, and C is very related to B. Thus, to efficiently program a way to find related ideas, we can find the categories or prominent ideas related to the idea, and then look for other ideas that fit in that category or are related to the prominent idea. We can hence think of categories as connectors between ideas. Note that an idea can belong to more than one category.