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'.
Proper nouns are the names of specific people, places, organizations, things, and ideas, and should always be capitalized. Common nouns name general people, places, things, ideas, etc. and are not capitalized. The following table lists proper nouns and related common nouns.