So What Are You Smuggling

Tuan comes up to the border between Vietnam and China on his bicycle. He has
two large bags over his shoulders. The guard stops him and says, "What's in
the bags?"
"Rice," answered Tuan.
The guard says, "We'll just see about that. Get off the bike." The
guard takes the bags and rips them apart; he empties them out and finds nothing
in them but rice. He detains Tuan overnight and has the rice analyzed, only to
discover that there is nothing but pure rice in the bags The guard releases Tuan,
puts the sand into new bags, hefts them onto the man's shoulders, and lets him
cross the border.
A week later, the same thing happens. The guard asks, "What have you
got?"
"Rice," says Tuan.
The guard does his thorough examination and discovers that the bags contain
nothing but rice. He gives the sand back to Tuan, and Tuan crosses the border on
his bicycle.
This sequence of events if repeated every day for three years. Finally, Tuan
doesn't show up one day and the guard meets him in a noodles restaurant in
Vietnam.
"Hey, Buddy," says the guard, "I know you are smuggling
something. It's driving me crazy. It's all I think about..... I can't sleep.
Just between you and me, what are you smuggling?"
Tuan sips his beer and says, "Bicycles."