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'.
Capitalize proper nouns. Proper nouns name specific people, places, and things. Examples of proper nouns are January, China, and Declaration of Independence. Words like month, country, and document are not proper nouns because they do not name specific nouns.