A youngster 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 the youngster.

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 the youngster overnight and has the sand analyzed, only to discover that there is nothing but pure sand in the bags.

The guard releases the yo, puts the sand into new bags, hefts them onto the younster's shoulders, and lets him cross the border.

The next day, the same thing happens. The guard asks, "What have you got?" "Sand," says the youngster.

The guard does his thorough examination and discovers that the bags contain nothing but sand.
He gives the sand back to the youngster and he crosses the border on his bicycle. This sequence of events is repeated every day for a year. .

Finally, the youngster 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?"

The youngster answers, "Bicycles."