Here's why:
* Common nouns refer to general categories of people, places, or things. They are not capitalized unless they start a sentence or are part of a proper noun.
* Proper nouns are specific names of people, places, or things. They are always capitalized.
Examples:
* Common noun: geyser, dog, city, mountain
* Proper noun: Old Faithful, Fido, New York City, Mount Everest
Since "geyser" refers to a general type of hot spring, it is a common noun.