HOW MANY WAYS can you make a tennis shoe, or tell a girl you love her?

The answer is that nobody knows, because people keep thinking up new wrinkles for athletic shoes, new ways to light the lamp of love and new ideas for just about everything under the sun or moon that you can make, use, read, listen to or look at.

The range of human ingenuity and concupiscence is unlimited, judging from an effusive new exhibit at the Library of Congress. For 200 years new designs and new literary and musical expressions have been pouring into the patent and copyright offices. More than 28 million have been registered, and the rate of applications is increasing.

There's never been an exhibit quite like this one at the library, and it's making quite a hit with visitors. There are cranks to turn, opinions to register, videotapes of Hollywood hits and inventor interviews. You can even follow the bouncing ball, or try to: a resiliency tester constantly drops heavy steel balls onto a row of tennis-shoe innersoles, leading to friendly arguments among the onlookers over whether the rebound was higher from Brand X or Brand Z.

It's a lot of fun -- if you catch the displays between their frequent breakdowns -- and accomplishes the serious purpose of showing how patents and copyrights stimulate creativity by protecting the intellectual property rights of innovators. There wouldn't be much incentive to build a better mousetrap or write the Great American Novel if anyone could come along and copy them for free.

New technology brings new problems: A cancer researcher recently was granted a patent on a better mouse, a strain into which he introduced an artificial oncogene. Computer companies sue each other over alleged pirating of the "look and feel" of programs. Ideas flash round the world while the law plods along in their wake.

One curious lapse mars the exhibit. Although humorist Samuel L. Clemens is cited twice, no mention is made of how he beat the system. When his copyrights began to run out, Clemens registered his pen name of Mark Twain as a trademark, which never lapses so long as it's kept in use. Although his writings long since passed into the public domain, they may not be reprinted under the byline Mark Twain without permission of and payment to his heirs and assigns.

The law was changed to keep other authors from following his example, but no doubt Sam Clemens still is savoring his eternal triumph in literary heaven, or wherever.

AMERICA CREATES: Two Hundred Years of Patents and Copyrights -- Through June 15 at the Jefferson Building, Library of Congress. 101 Independence Ave. SE. 707-6400. Open 8:30 to 9:30 Monday through Friday, 8:30 to 6 weekends. Good wheelchair access (ground-floor exhibit). Metro: Capitol South or Union Station.