copyright 2009, The Washington Post WASHINGTON -- What do you think of when you hear the words 'the Smithsonian'? The National Museum of the American Indian? The merry-go-round on the Mall? The inappropriate use of government funds? What about: Illegitimate Englishman leaves the equivalent of $50 million to $100 million in his will to a place he had never visited and had no connection to? 'There's a lot we don't know about James Smithson,' guide John Heyob tells the group standing in front of Smithson's tomb as he gets ready to lead a tour of one of the Smithsonian Institution's most recognizable buildings. The Smithsonian Castle: You see it in photos, you scurry past it on the way to the Air and Space Museum, you lunch in its garden in the summertime. But the castle itself is worth exploring, too. The most noteworthy item is, in fact, Smithson's tomb. Smithson died and was buried in Italy in 1829. His remains came to Washington in the early 1900s. There he lies, the reason these artifacts -- the Hope Diamond at the Museum of Natural History, the Star-Spangled Banner at the Museum of American History -- are all here in one place. To think that, in 1835, some members of Congress didn't want to accept his money. President Andrew Jackson wondered if it was legal to take it. The funds initially went to Smithson's nephew. The will stated that if the nephew died without children, the money would go to the United States. Six years after Smithson died, so did his childless nephew. Smithson's money came to the states in the form of 105 bags of gold, which were then melted down in Philadelphia. Those are some of the fun facts your guide will tell you. The castle houses pieces from 16 Smithsonian museums. (It's a good place to figure out where to go next.) You'll also see a model of the original design of the castle that was found in the attic in 1965 and is one of the oldest building models in the United States. A couple of impressive-looking chairs from the office of President Lincoln's secretary of war, Edwin Stanton, sit in Sherman Hall, the former library in the castle. Stop and catch Ben Stiller's very funny narration of the Smithsonian orientation film after the tour. Smithson's vague and laconic will instructed that his money be used 'for the increase and diffusion of knowledge.' By taking this 45-minute tour of the castle, you'll be doing just that. Where is it? 1000 Jefferson Dr. SW, Washington. Tours are Saturday at 9:30 and 10:30 a.m. and Sunday at 10:30 a.m. Easily accessible by the Smithsonian Metro stop. How much is it? Free! If you have more than three hours. Grab a coffee at the castle cafe and then head next door to the Freer Gallery of Art. 