A customs officer at the Mexican border noticed a man coming across one day on a bicycle with two small sacks tied to the handlebars. He naturally got suspicious and asked him to open the sacks, but when he did he found nothing but sand.
Each time he'd stop the bicycle and open the sacks, and he'd find only sand. He had the sand analyzed at the lab and looked at the sack under a microscope he could never find anything wrong. This went on every day for the next month.
After the officer retired, he ran into the bicyclist in a restaurant in Tijuana. After some small talk he said, "Come on. I know you were smuggling something all that time. I won't tell. I'm just curious. What was it?" The other man said, "Bicycles."