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," answers 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 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.

A week later, 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,
who crosses the border on his bicycle.

This sequence of events is repeated
every week for three years.
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 coke and says,
"Bicycles."