"Look! This is an example!" Joe said.
Lucy said, "Duh. You said that already."
A proper noun is "a noun that is not normally preceded by an article or other limiting modifier" (m-w.com). It is used to denote a particular person, place, or thing.
The Landmark Inn
I love talking about the time that I went on vacation to that place that I love.
Doctor House is always right.
House, a doctor of medicine, knows it is never lupus.