"What's in the bag" joke

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 bike, 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 bike
across the border.
Two weeks later, the same situation is repeated...
"What have you there?"
"Sand"
"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 the fellow, "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."