There are a million nuts out there obsessed by the idea of building a better snigglefritz and growing rich and famous. Occasionally, as an exhibit at National Geographic headquarters shows, one does come up with the real McCoy.

The real real McCoy was an automatic locomotive oiler patented by Elijah McCoy, son of slaves, in 1872. Mechanics would check each new engine that came into the shop to make sure it wasn't equipped with an imitation.

The exhibit, making its first stop on an 11-city tour, is sponsored by the Small Business Administration, which is why it features little guys rather than the corporate research centers where most modern inventions originate. All but two of the inventors featured received their patents before World War II. It does indeed describe "the role of small business innovation in America," but it's hardly reassuring.

But that is not to say it isn't fun. Most of the exhibits are "hands on," from the sound-powered telephone to the Moog Synthesizer. It's worth the trip just to finally be able to understand how zippers work. And just like so damn many of the real ones, the giant model zipper won't stay closed.

The exhibit will run through August 2 in Explorers Hall, 17th and M streets NW. The hours are 9 to 5 Saturdays and holidays, 10 to 5 Sundays and 9 to 6 weekdays. Free.