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Cool - in a way is a very common word among the Americans. This word can briefly tell one's feeling on something easily. Therefore it is a trend for more and more Americans to use this word in their daily life.
Great - is a very useful word. When you try to be polite you can always say this word to other people about their look, work or behavior. Most foreigners are trying to speak "good" word to other people in their daily life, therefore the word "Great" is a word that you can use without making anyone angry, or show your appreciation on other people's work or behavior.
You know - is a term that is being used as a meaningless beginning of a conversation. When people say "you know", most of the time they are not saying that "you really know". This is just like other meaningless word like "well". In fact, it is not very polite to always use the word "you know" in a conversation, because people will find it annoying that you always imply that you are more clever than the person you talk to.