A man on a bike, carrying two sacks on his shoulders, was stopped by a guard while crossing the US-Mexican border.
"What's in the bags?" asked the guard.
"Sand," the cyclist replied.
"Get them off. We need to take a look."
The guard emptied the bags and found out they contained nothing but sand. The man reloaded his bags and continued across the border. A week later, the same man was crossing again with two more bags. The guard demanded to see them, and again they contained nothing but sand. This continued every week for six months, until one day the cyclist failed to appear. A few days later, that same guard ran into the cyclist in the city.
"Hey, where have you been?" the guard asked. "You sure had us wondering! We knew you were smuggling something across the border. So tell me and I won't say a word. What was it?"
The man smiled and told him the truth. "Bicycles!"