Juan comes up to the Mexican border on his bicycle.
He's got two large bags over his shoulders. The
guard stops him and says, "What's in the bags?"
"Sand," answered Juan.
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 sand. He detains Juan overnight and has the
sand analyzed, only to discover that there is nothing but pure sand in
the bags. The guard releases Juan, puts the sand into new bags, hefts
them onto the man's shoulders, and lets him cross the border.
The next day, the same thing happens.
The guard asks, "What have you got?"
"Sand," says Juan.
The guard does his thorough examination and discovers that the
bags contain nothing but sand. He gives the sand back to Juan,
and Juan crosses the border on his bicycle.
This sequence of events is repeated every day for
a year. Finally, Juan doesn't show up one day and
the guard meets him in a cantina in Mexico.
"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?"
Juan sips his beer and says, "Bicycles."