What defines cool?
A new exhibit at the Smithsonian Institution tries to answer this question with a collection of portraits of the "coolest" writers, musicians, actors and personalities in U.S. history.
According to the museum, the word "carries a social charge of rebellious self-expression, charisma, edge and mystery."
"American Cool" will be on display at the Smithsonian's National Portrait Gallery from Feb. 7 to Sept. 7, 2014.
In this photo, James Dean, 1954