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 the name given to something to make it more specific (e.g., Johnathan, Ollie, London, Monday). Proper nouns are written with capital letters regardless of where they appear in a sentence. Proper nouns contrast with common nouns, which are the words for something (e.g., boy, dog, city, day). Common nouns are written with a capital letter only when they start a sentence. Remember, everything we can see or discuss is represented by a word that names it. That word is called anoun.