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.
   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."