A customs officer observes a truck pulling up at the border.
Suspicious, he orders the driver out and searches the vehicle. He pulls off the panels, bumpers, and wheel cases but finds not a single scrap of contraband.
Still suspicious but at a loss to know where else to search, he waves the driver through.
The next week, the same driver arrives. Again the official searches, and again finds nothing illicit.
Over the years, the official tries full-body searches, X rays, and sonar, anything he can think of.
Each week the same man drives up, but no mysterious cargo ever appears, and each time, reluctantly, the customs man waves the driver on.
Finally, after many years, the officer is about to retire. The driver pulls up. “I know you’re a smuggler,” the customs officer says. “Don’t bother denying it. But [darned] if I can figure out what you’ve been smuggling all these years. I’m leaving now. I swear to you I can do you no harm. Won’t you please tell me what you’ve been smuggling?”
“Trucks,” the driver says.