I think my favorite is dumb because of the time and place. I was in my oncologist's office, trying to explain to an endless array of bean counters how I would pay for my treatment without insurance. I filled out lots of forms, and listed my job as self-employed, because I'm an artist. When my copy of what was entered into the computer was printed out, it said I was unemployed, so we had to go three rounds about the fact that no insurance didn't mean no job.
Finally I get into the office to see the doctor, and his nurse started asking me the same questions I'd already answered on the forms, which were sitting right in front of her, and entering them into yet another computer. When she got to my job, she asked me what I did, and I said, "I'm an artist".
She looked at me like I had two heads, and asked, "Do you make money for that? I mean, do people pay you to do that?"
Yes, they do. But she just wouldn't believe it, and kept asking how that worked. Finally, I'd had enough of the quizzing, and said, "Listen. I own a house. I pay a mortgage and bills. I pay taxes. I have a retirement fund. I just have an unusual way of making the money that makes that happen. OK?"
End of discussion. Everyone in the office treated me much nicer after that.