A worker of a games company poses with placards depicting a "royal baby" near the St. Mary's Hospital Lindo Wing in London on Thursday. While Buckingham Palace has been mum on the subject, Saturday was rumored to be the official due date for the child who will become the third in line to the British throne. Lefteris Pitarakis/AP hide caption

A portrait of Mary Walker from the National Archives. Mathew Brady/NARA hide caption

He's legend now, but Johnny Appleseed was as odd as his myth. Time & Life Pictures/Getty Image hide caption

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

A "featherie" golf ball from the 1800s. iStockphoto hide caption

Leon de Lunden of Belgium won the live pigeon shooting event at the 1900 Olympics in Paris — the only time in Olympic history when animals were killed on purpose. Popperfoto/Getty Images hide caption

A photograph of the Watergate complex that was used as an exhibit in the trial of G. Gordon Liddy. National Archives hide caption

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

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

The metric system is based on the number 10. Sounds simple, but the U.S. is having none of it.

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

Two men operate the enormous UNIVAC (UNIVersal Automatic Computer) in 1960. Hulton Archive/Getty Images/Life hide caption

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