Invisible Nerd Tattoo

On the last leg of my flight from San Diego to New Hampshire on Wednesday, I boarded the plane, settled into my middle seat, and began to read the magazine I had purchased (Scientific American). After a few minutes, the woman whose seat was next to mine arrived, stowed her luggage, and sat next to me. She immediately looked over her shoulder at what I was reading, looked up at me, and says, "Engineer eh?" I was floored! How did she know I was an engineer from my magazine? I wasn't reading Engineers-Are-Us or anything. I'm thinking that that at some point when I was passed out in the Mad Lab at Syracuse from exhaustion brought on by a double or tripple all-nighter I was branded with some sort of tattoo across my forehead that simply reads, "Engineer." Maybe even "Enginerd." Of course this tattoo is invisible to all engineers or is in some color that engineers can't distinguish (sort of how dogs supposedly can't see in color). I'm sort of upset that my non-engineer friends never told me about this marking of mine. Perhaps they were simply trying to help me lead as normal a life as possible.