An agent is checking out a Mexican who was riding his bike across the border one day. The agent doesn't suspect anything and lets him go through.

The next day the same Mexican is seen by the same agent riding his bike and is checked out and lets him through.

After the third day, the agent starts to get suspicous, and asks another agent to check him out more thoroughly, but still finds nothing illegal on him.

After 20 days of this, the agent pulls the Mexican over to the side and tells the Mexican that he knows he's smuggling something in and out, but can't figure out what it is. The agent tells him that he won't say anything to the authorities, if he just told him what it was. The suspense was killing him.

The Mexican says bicyles.