The caffeine content of coffee varies greatly, depending on the beans, how they're roasted, and other factors, but the average for an 8-ounce cup is about 100 milligrams (mg).
No, 'cat' is a common noun, a word for any cat of any kind. A proper noun is the name of a person, a place, a thing, or a title; for example: Cat Deeley, host of TV series 'So You Think You Can Dance'.
A proper noun is a specific name given to a person, place, thing, or idea. It is its official name. For example, Lake Michigan, 28th Street, John Smith, and Jane Doe are all proper nouns. Proper nouns are always capitalized, regardless of how they are used.