Alexander Gardner's original caption for this image: "A Lone Grave, on Battle-field of Antietam." Alexander Gardner/Library Of Congress hide caption

itoggle caption Alexander Gardner/Library Of Congress

Swiss guards march prior to a swearing-in ceremony at the Vatican on Sunday. This year, 26 Swiss men are joining the oldest standing army in the world, swearing to give up their lives to protect the pope. The ceremony is held every May 6 to commemorate the day in 1527 when 147 Swiss Guards died protecting Pope Clement VII during the sack of Rome. Alessandra Tarantino/AP hide caption

itoggle caption Alessandra Tarantino/AP

President Calvin Coolidge (right) with the national Christmas tree on Dec. 24, 1923. National Photo Company/Library of Congress hide caption

itoggle caption National Photo Company/Library of Congress

An engraved illustration of fighting during Shays' Rebellion of 1786, circa 1850. Hulton Archive/Getty Images hide caption

itoggle caption Hulton Archive/Getty Images

The horses hit the 1-mile mark during the 1901 Kentucky Derby. Library of Congress hide caption

itoggle caption Library of Congress

Lithograph of the 1861 bombardment of Fort Sumter in South Carolina's Charleston Harbor. Currier & Ives/Library Of Congress hide caption

itoggle caption Currier & Ives/Library Of Congress

Who Knew? For more than 20 years, research librarian Kee Malesky has answered questions for NPR reporters, editors and hosts. She has compiled some of her favorite bits of "inessential knowledge" — such as which building did Elvis leave last? -- in a new book, All Facts Considered. Robert P. Malesky hide caption

itoggle caption Robert P. Malesky