You’ve made it this far — through security at the overseas airport, the whimpering baby in Seat 22E, the neighbor’s seat back in your lap — and you have just one more hurdle to clear: the U.S. Customs Declaration form. The double-sided slip of paper is basically all that stands between you and home, your dog and the pile of unopened mail. Sharpen your pencil, and your responses.

With only 15 questions, including 10 that you could answer under the influence of Ambien, the grayish-blue form seems easier to fill out than a Cosmo quiz. No self-analysis required, just the cold, hard facts: name, birth date, street address, country of residence, countries visited, etc. Until — dramatic drumroll, please — you reach the lower half of the card. You know, the part that asks what you bought during your travels, how much you spent and whether you’re carrying any animals, vegetables, snails or soil.

These aren’t trick questions, but they are tricky. Even the most seasoned traveler might not think to declare a banana swiped from the plane or to categorize a snakeskin belt from Italy as a “wildlife product.” Or, most shocking, to list every single ticky-tacky item you purchased, including the Prince William and Kate Middleton tea towels from London.

“You need to be as specific as possible,” says Christopher Downing, a supervisory U.S. Customs and Border Protection (CBP) officer at Dulles International Airport. “The more specific you are, the less time you’ll spend here.” Here, by the way, refers to the Customs and Border Protection area that all passengers, foreign and U.S.-born, must pass through post-plane and pre-release into the United States.

Yes, reentering the country can often be a speedy process, as quick as a Jiffy Lube oil change. But make a few missteps and prepare for some detours. The Customs inspectors will show you the way.

Customs Declaration card (twp scan)

To understand the multistep procedure and sneak a peek behind the Customs curtain, I spent a day and a half at Dulles. I observed some of the 250 federal workers as they interviewed passengers, inspected their luggage, confiscated goods and eventually lifted the garage door to the good ol’ U.S.A. Standing at the front lines, I learned how we the people can be better (returning) patriots, starting with Question 11: “I am (We are) bringing. . . .”

Brace yourselves for the crowds, folks. Between November 2010 and October 2011, more than 3.3 million passengers flew into Dulles from foreign lands. Between the peak hours of 1:30 and 5:30 p.m., Customs officials may interview 2,500 to 5,000 international visitors and U.S. residents flying in from Brussels, Frankfurt, Geneva, Jeddah, Moscow, Paris, Riyadh, Rome and Vienna — and that was just one weekday afternoon this pastfall.

The first requirement: Fill out the declaration card, preferably before you join the rivulet running through the primary screening area.

“Anything that you acquired outside the United States, you must declare,” Downing told me. “Even if you found it or it fell out of a truck. Nine times out of 10, it’s allowed. But just tell us.”

I tested him on this rule of thumb, tossing out such examples as T-shirts, cheap souvenirs and food lifted from an in-flight meal. I received affirmative on all. In addition, I learned that Customs likes specifics. A bracelet is not just a bracelet, for instance, if it contains seeds, coconut shell or other forms of living material. Now it’s a plant product, which requires a check in the “yes” box under Question 11a. In the same vein, shoes and belts aren’t just shoes and belts if they’re made from animal skins. See the wildlife product category; you know what to do.

“They think that it’s just a purse,” said Kelly French, a U.S. Fish and Wildlife Service inspector, “but it’s about how that purse is made of snake.” (For the record, cobra is allowed; python is banned.)

The declarations are crucial on two fronts. CBP needs to know the amount you spent abroad to calculate the exemptions (up to $800 in items per person) and assess any additional payments. The second reason is a two-parter, to protect endangered creatures worldwide and to safeguard the country from disease and the destruction of flora and fauna. The biggest concerns are foot-and-mouth disease, mad cow disease, avian influenza and exotic Newcastle disease, an avian virus. Bugs, parasites, pests and other uninvited guests can catch a ride on fruits, vegetables, plants, meats and other organic matter. Banana and Beef Airways really packs them in.

“Americans are generally not honest about food, especially when they come across a good sausage from Italy or Germany,” said Michael Harris, an officer since 1999. “But if you ask them the right way . . . ”

The “right way” is not standardized; it varies by individual and situation. Customs officials, who undergo six months of training, are keen readers of body language and mannerisms. To the sharp ear, a hesi­ta­tion or a sigh can be as revealing as an open confession; to the astute eye, nervous tapping and a wandering gaze can betray obfuscation.

Passengers who pass the first line of defense move on to the baggage carousel to collect their luggage. Those who fail either have to hang back (such as the French woman who had only the P.O. box of the friend she was visiting in Gaithersburg and had to chase down a physical address) or are red-flagged for secondary screening or for more intense questioning.

Throughout the process, the inspectors snoop for clues. Some passengers are sly, hiding sausage in an empty orange juice container, for instance. Others, though, are the opposite of smart.

One employee told me about a woman who carried a transparent bag that exposed the food product inside. An easy bust. Another traveler told the officer that authorities in Peru, her departure point, had confiscated parrot feathers at the airport. Her confession tipped inspectors to the possibility that she might be harboring other illegal objects. Their hunch was correct. In her luggage, they found a shark’s tooth necklace (allowed), an ocelot skull (not) and two belts made of the wildcat’s fur (definitely not).

In another case, French nabbed a woman for the elephant ivory bracelet dangling from her wrist. The strategy of wearing the item in the hopes that officials might not notice or might assume that the object predated the trip is foolhardy. First, the sharp-eyed experts are checking you out. Second, they’re authorized to confiscate contraband purchased on previous travels and even items that have been handed down as heirlooms. (Advice: Leave the precious or questionable baubles at home.)

“In the first 10 seconds of the interview, I can tell if they are feeding me a line . . . or are being truthful,” said enforcement officer Brian Keys. “Good people can’t lie good.”

Of course, sometimes good people don’t realize that they’re being bad. Like the couple who wrapped up their apples and sandwiches from the plane ride, hoping to snack on them later. Or my innocent self.

In the baggage collection area, Thomas the food-sniffing beagle approached me, then flopped down at my feet. He’d caught the residual scent of a banana I’d eaten on the drive to the airport.

“Thomas’s job is to locate the targets,” said Kristi Currier, his handler. “He can’t tell what country it’s coming from and if it’s banned, but when he comes close to it, he sits down.”

Currier totes around a bag of anecdotes from her adventures with Thomas. There was the man Thomas outed for carrying apples in his backpack, and the woman who greeted the pup excitedly, having originally met the canine cop years ago, when he busted her for transporting her grandmother’s meatloaf.

The dog’s discoveries are sometimes seasonal, such as chestnuts and holiday meats over Christmas, moon cakes (which feature uncooked eggs) for the Asian New Year, and Dutch tulip bulbs in the spring (permitted with the proper certification).

“The idea is to leave it on the plane,” said Currier, “or give it to Agriculture when you get off.”

Now, if you are imagining Customs officials gleefully picnicking on the confiscated goods — say, a spread of Italian sausage, a slice of French soft cheese and fresh Turkish figs — banish your thoughts to the incinerator. Because that’s where the forbidden items go. (Drugs are held in a secret place as evidence, then destroyed in another mysterious location. No matter how many times you ask in so many variations, no one’s talking.)

In a small room with a fridge and posters of parasites, Jeff Davis, an Agriculture specialist, was lugging around a bucket of okra. The green veggies, uncovered in a Gambian woman’s luggage, were speckled with white goo. I assumed that the flecks were fungus until Davis corrected me: The passenger’s hair cream had exploded in her bag.

Poking around the lablike space, I asked Davis what lived inside a giant trash bag secured with yellow tape. He ripped open the sack and showed me fragrant wood pieces from Sudan, which locals burn as incense. In another bag, he pulled out dried lemons from Iran. I picked one up for a whiff, then stopped myself. I didn’t want to become anyone’s conveyance.

Some confiscated matter is obvious, such as giant African snails (prodigious procreators and no natural predators) and chicken feet from China (avian flu). But what about the ornately decorated ostrich eggs from South Africa? Though harmless to the eye (and to the stomach, as most of us don’t dine on artwork), the eggs can potentially harbor the avian flu virus. “There is no way to cook it out,” said Davis, as he showed me an egg expertly painted with images of African game.

A number of animal- and plant-based items aren’t banned outright but require additional qualifiers. For example, meats from Iceland, New Zealand and Australia are permitted, but those from much of Europe are banned. The difference? The former nations have no reported cases of foot-and-mouth disease. To be prudent, the agency recommends that you skip all beef mementos.

When goods are seized, travelers not only lose their souvenirs (Davis and his colleagues can recount many a tale involving tears or tantrums), but they also risk paying a fine. The first offense costs $300, the second $500, the third $1,000. Do it again and the State Department can curb your travel privileges. Makes you think twice about bringing in those yummy beef candies from China.

Honesty, however, can save you some Franklins. “All you have to do is declare it,” said Davis, “and any fines are waived.” The officials also factor in agreeable behavior.

In all fairness, sometimes the rules are fuzzy or complicated. For example, you can bring in unlimited quantities of red caviar and 125 grams of sturgeon caviar but not beluga, which is endangered. No on African elephant ivory but yes on warthog and hippo ivory (as long you comply with foreign laws). Dried fruit is permissible, as are juices and souvenirs constructed of processed coconut husks and seeds.

“You have to be very careful bringing in animal products because there are many, many protected species,” said French. She said to check with the CBP or a wildlife specialist before your trip. If you’re already abroad, ask the vendor about the product and obtain any relevant paperwork regarding its origins. If doubt remains, pass on the purchase.

The fish and wildlife experts base their decision on a combination of policies, including local laws and regulations, and the Convention on International Trade in Endangered Species, a multi-country conservation agreement. However, even I, an untrained observer, sensed that the proteins tucked inside a suitcase from Cameroon were not permitted.

While I was inspecting French’s box of banned finds, a young woman started to open her bags beside me. I put down the pair of Italian designer heels made of tagu, an endangered lizard, to watch.

Peeling back layers of clothes, an agriculture pro discovered parcels of meat wrapped in newspaper: cow skin, beef and bush meat topped with a fish swallowing its tail. The smell was overpowering, and the sight of half-cooked brown chunks squelched any appetite for lunch. The woman, clearly upset, muttered that she’d spent $100 on the meats.

But she didn’t protest as agents seized the goods. She zipped up her luggage and rolled the lightened load through the doors, and into the United States.