Juan the smuggler

Juan comes up to the Mexican border on his bicycle. He has two large bags over his shoulders. A 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, lifts 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 repeats every day for three years. Then one day, Juan doesn't show up. The guard meets up with him in a cantina in Mexico.
"Hey, buddy," the guard says, "I know you're 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?"
"Bicycles," Juan says