Here's why:
* Abstract nouns represent concepts, ideas, qualities, or states of being. They are intangible and cannot be physically touched or observed.
* Concrete nouns represent physical things that can be seen, touched, tasted, smelled, or heard.
A smile is an expression of happiness, joy, or amusement. It is not a physical object but rather a mental state that is expressed through facial muscles. You can't hold a smile or put it in your pocket.
Therefore, "smile" is an abstract noun because it represents an idea or emotion rather than a tangible object.