President Barack Obama: then and now (Image source: The Washington Post)
It's common knowledge that 1600 Pennsylvania Avenue is one of the quickest routes to a lined face and grayed temples, and yet the transformation is nevertheless fascinating to watch.
The Washington Post pieced together 99 photos of President Barack Obama that chronicle how his time in the White House has aged him -- the lines, the deeper creases, the salt-and-pepper hair (and how some of that color will sometimes mysteriously become more uniform).
There's even a cool interactive tool that allows you to slide the years on or off (shown above).
President Barack Obama on the night of his Inauguration, January 2009. (Image source: The Washington Post)
President Barack Obama delivering his first State of the Union, January 2010. (Image source: The Washington Post)
President Barack Obama after the shooting of Rep. Gabrielle Giffords, January 2011. (Image source: The Washington Post)
President Barack Obama after the Benghazi, Libya attacks, September 2012. (Image source: The Washington Post)
The "why" part is easy: unending, unfathomable stress.
"You see it over a term,” Ronan Factora, a geriatric medicine specialist at the Cleveland Clinic told the Post. “It’s a good study of chronic stress on a person’s overall health.”
See the full progression at the Washington Post, including features on former Presidents George W. Bush, Bill Clinton and George H.W. Bush.