Here's why:
* Concrete nouns refer to things that you can experience with your five senses (sight, touch, smell, taste, hearing). A detour is a physical path or route you can see and travel on.
* Abstract nouns refer to ideas, concepts, qualities, or feelings. While the concept of taking a different path could be abstract, the physical route of a detour is tangible.
Let me know if you have any other words you'd like to classify!