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Mount Rushmore National Memorial, near Keystone, South Dakota, is a monumental granite sculpture that represents the first 150 years of the history of the United States with 60-foot sculptures of the heads of former U.S. presidents George Washington, Thomas Jefferson, Theodore Roosevelt, and Abraham Lincoln. The heads symbolize, respectively, four significant American ideals: independence, democratic process, leadership in world affairs, and equality.

The Penrose triangle, also known as the tribar or impossible triangle, is an impossible object, first created by the Swedish artist Oscar Reutersvärd, popularized by Roger Penrose, and later featured prominently in the works of artist M. C. Escher. Impossible figures, such as the impossible cube and blivet, are a special class of ambiguous figures in which parts of the picture that are not ambiguous are drawn in incompatible perspectives.