Mwangi comes up to the Kenyan border on his bicycle. 
He has two large bags over his shoulders.  The guard 
stops him and says, "What's in the bags?"

"Sand" answered Mwangi. 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 Mwangi 
overnight and has the sand analyzed, only to discover 
that there is nothing but pure sand in the bags. The 
guard releases Mwangi, 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 Mwangi. The guard does his thorough examination 
and discovers that the bags contain nothing but sand. He 
gives the sand back to Mwangi, and Mwangi crosses the 
border on his bicycle. 

This sequence of events is repeated every day for three 
years.  Finally, Mwangi doesn't show up one day and the 
guard meets him in a Café in Nairobi. "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?" 

Mwangi sips his beer and says, "Bicycles." 

Anon