Juan comes to US/Mexico border on a cycle with 2 large bags over his shoulders.
The guard stops him and says, “What’s in those bags?”
“Sand,” answered Juan.
The guard says, “We’ll see about that. Get off the bike.”
The guard takes the bag 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 let’s him cross the border.
A week later the same thing happened.
The guard asks, “What have you got?”
“Sand,” says Juan.
The guard does his through examination and discovers that the bag contains nothing but sand. He gives the sand back to Juan, and Juan crosses the border on bycycle.
This sequence of events is repeated every day for 3 years.
Finally, Juan doesn’t show up one day and the guard meets him by coincidence. “Hey buddy,” says the guard, “I know you are smuggling something. It’s driving me crazy. It’s all I think about and I can’t sleep. Just between you and me, what are you smuggling.”
Juan sips his corona and says, “Bicycles.”