Juan and the sand

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 analysed, 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.

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, and Juan crosses the
border on his bicycle.

This sequence of events if repeated every day 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 beer and says, "Bicycles."