Here's why:
* Common nouns refer to general, non-specific people, places, or things. "Tour" can refer to any organized trip or journey, not a specific one.
* Proper nouns are specific names. For example, "The Grand Tour" or "The Louvre Tour" would be proper nouns.
Examples:
* I took a tour of the city. (common noun)
* The tour guide was very knowledgeable. (common noun)
* We booked a tour to Europe. (common noun)
Let me know if you have any other questions!