Customs agents aren't hired for a sense of humor. When driving a semi I crossed from Canada to US almost every week for a year at a crossing at Massena NY. Knew most of the agents by name and never had a problem till I made a smart a____ remark about contraband. Everything came out of the cab of the truck, trailer and my pockets. It took me almost two hours to clear customs. Next week the sane thing. The following week they asked if I learned my lesson. "Yes Sir" was the proper answer. After that I never had another problem.
Be polite, don't volunteer anything, and tell the truth, and you'll be OK.
And no jokes about contraband!