The longer we hold information in short-term memory, the more likely it will transfer to the third and largest storage area long-term memory, our permanent knowledge base, which is unlimited. Information in long-term memory is categorized by its contents, much like a library shelving system that allows us to retrieve items easily by following the same network of associations used to store them in the first place. In each, mental strategies can be used to manipulate information, increasing the efficiency and flexibility of thinking and the chance that information will be retained.