I just like to give these people something different in their day. A cashiers job can become very dull and boring. I just like to add some surprise to their day. Most of thime they laugh when they realize I am kidding.
Once, when I was working as a chemist in a lab, we had a new guy get hired. He was a PhD and had taught in high schools for awhile. The first day on the job he got a phone call from his girlfriend on his cell. I was just walking by, not planning anything, and I noticed that his gf didnt seem to believe he had gotten a new job. I walked up close and said, "Hey Tim, are you finished cleaning the bathrooms? we need your plunger and mop bucket back" He capped the phone and said (with big eyes) 'you son of a bitch!' I walked off laughing while she now believed he was working he was still trying to convince her that he was a chemist!