I have had that question posed to me by preschool children before and again last night when out shopping. Kids are notoriously curious, open and out spoken and I consider it a reasonable question that mostly amuses me. I have replied with, "I used to be more like a boy and now I am more of a girl." One little girl, whose family I know, was not satisfied with that and had more questions that I would not be comfortable answering and then started a repetitious chant of "you're a boy, you're a boy.." This was in a public place and, in my experience, kids have more energy and tenacity at a game that we won't win. They also can be expected to get loud and we may not respond in kind. I just smiled and talked to her mom.
Beyond simple answers, we may risk offending parents and creating a scene where nobody feels good. I like to think there are creative ways to open the education door a bit without inciting offense.
Got any one or two line responses that have worked for you?