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Gerald Herbert, Associated Press
At age 47, Barack Obama is the fifth youngest president, and the fourth youngest elected.

Here are 20 things, from Ian Randal Strock's "The Presidential Book of Lists" you may not have known about U.S. presidents:

1. Four presidents never held an elective office before being elected: Zachary Taylor, Ulysses S. Grant, Herbert Hoover and Dwight D. Eisenhower.

2. William Henry Harrison caught pneumonia while giving a long inaugural speech in bad weather on March 4, 1841. He died just after midnight on April 4, 1841, having served one month.

3. David Atchinson was technically president for one day in 1849, when the inaugural ceremony fell on a Sunday, and President-elect Zachary Taylor refused to accept the oath of office until the next day. Atchinson was president pro tempore of the Senate, and next in line.

4. Recent evidence suggests that Chester Arthur may have been born in Canada, and thus technically not eligible to serve as U.S. president. His birthplace was always claimed as Vermont. But since he's long dead, the point is moot.

5. The name James was shared by six Presidents: Madison, Monroe, Polk, Buchanan, Garfield and Carter.

6. Andrew Johnson had no formal education. He taught himself to read as an adult.

7. Only four presidential candidates have won more than 60 percent of the popular vote.

8. The most prolific president was John Tyler, who fathered 15 children (eight sons and seven daughters) with two wives.

9. Ten presidents were Episcopalians: George Washington, James Madison, James Monroe, William Henry Harrison, Zachary Taylor, Franklin Pierce, Chester Alan Arthur, Franklin Delano Roosevelt, Gerald Ford and George H.W. Bush.

10. Ulysses S. Grant changed his name from Hiram Ulysses Grant because he did not want his West Point gear branded with the initials HUG.

11. Grover Cleveland's second wife was 27 years younger than her husband and had been his ward since the age of 11, when her father, Cleveland's former law partner, died.

12. Abraham Lincoln is the only president to have worn a beard with no mustache.

13. John Quincy Adams and Theodore Roosevelt were 14th cousins, once removed.

14. The most popular college presidents-to-be graduated from is Harvard: John Adams, John Quincy Adams, Theodore Roosevelt, Franklin Delano Roosevelt and John Fitzgerald Kennedy. Rutherford B. Hayes graduated from Harvard Law School, and George W. Bush graduated from Harvard Business School.

15. James Madison, who nearly single-handedly wrote the Constitution, was 5 feet 4 inches tall, making him the shortest president to date.

16. James Knox Polk died three months after leaving office, on June 15, 1849. His wife, Sarah Childress, outlived him by 42 years, 60 days, dying on Aug. 14, 1891.

17. Woodrow Wilson and his second wife, Edith Bolling Galt, are the only presidential couple to share a birthday and death day. He was born on Dec. 28, 1856. She died on what would have been his 105th birthday, Dec. 28, 1961.

18. Virginia was the birthplace for eight of the presidents, including four of the first six: George Washington, Thomas Jefferson, James Madison, James Monroe, William Henry Harrison, Zachary Taylor, John Tyler and Woodrow Wilson.

19. Gerald Ford's birth name was Leslie King Jr. His name was changed when he was adopted by a stepfather. The same thing happened to William Jefferson Blythe, who became William Jefferson Clinton.

20. The average U.S. president:

Has a better than one in four chance of being named James or John

Has a better than one in three chance of having been born in Virginia or Ohio

Is 55 years, 117 days old on Inauguration Day

Has a life expectancy of 70 years, 245 days

Is 5 feet, 10 inches tall

Has 3.7 children

Has a 28 percent chance of being the firstborn child in his family

Has a 50 percent chance of being a lawyer

Has a 36 percent chance of having been a senator

Has a 69 percent chance of having served in the military

Has a four in seven chance of being a Republican

Has a one in three chance of being a Democrat

Has a 10 percent chance of being assassinated.