On the flight, a girl rested her dirty foot on the tray table of the empty seat. I asked her kindly to take it down—but in the end, I had to insist more firmly

Not long ago, I had a short flight—just two hours—but it left a memory I’ll never forget.

Everything started off normal: I settled into my aisle seat, the middle seat was empty, and by the window sat a young woman.

About ten minutes after takeoff, her behavior took a bizarre turn. First, she slowly slipped off her socks. Then, she …