A fellow tries to cross the Mexican border on a bicycle with two big bags balanced on his shoulders. The guard asks, "What's in the bags?"
The fellow says, "Sand!"
The guard wants to examine them. The fellow gets off the bicycle, places the bags on the ground, opens them up, and the guard inspects... only to find sand. The fellow packs the sand, places the bags on his shoulders, and pedals the bicycle across the border.
Two weeks later, the same situation is repeated...
"What have you there?"
"We want to examine."
... Same results... nothing but sand and the fellow is on his way again.
Every two weeks for six months the inspections continue. Finally, one week the fellow didn't show up. However, the guard sees him downtown and says to him, "Buddy, you had us crazy. We sort of knew you were smuggling something. I won't say anything - what were you smuggling?"
The fellow says, "Bicycles."