If you have ever traveled outside the United States, you encountered the U.S. Customs Service on your way back home. You probably are most familiar with the role of these uniformed men and women in checking that declaration form you filled out on the airplane and examining people's luggage to be sure nothing illegal is tucked inside. You may have also learned more about customs laws -- the hard way! Did you have to give up that fresh apple you brought along in your carry-on bag? Or maybe that new medication you brought back because it was available over-the-counter in your destination country (and unavailable without a prescription at home)?

In this edition of How Stuff Works, we'll answer these questions and others designed to help you prepare for your next international trip by explaining how customs works. Since customs laws vary so much from one country to the next, we'll focus on the U.S. Customs Service and some of the laws that govern what you can and cannot bring back into the U.S.. And if you thought that all U.S. customs officers do is stand around at airports or border stations and look through people's bags, you'll be surprised by the far-reaching activities of this government agency, which enforces 600 laws for more than 60 different government agencies and prevents thousands of cases of drug smuggling each year.