Kids are good for this. I was hanging out with a friend the other day in girl mode, although I don't pass for crap, and within 20 seconds of me walking through the front door my friends 4 year old daughter ran up to hug me, stopped, looked at me, and said "Hey, It looks like you have boobies." Mind you, multiple times now both I and her parents have attempted to explain my transition to her, but she just doesn't get it. So I just said "Well, I do have boobies"
"Why?" she said.
"Because some people grow them. You will too one day."
"It must be because you wear girls clothes."
"Sure, that must be it."
Thankfully she's young enough that she'll forget boy me a I get further in transition. Her older sister, however, wont, and firmly thinks I'm a freak. She's gonna be the one, who in 5 years, will probably tell some random passerby on the street "She used to be a boy." Sigh.
At least it happens to genetic girls too. My ex, who was born a woman, and has always presented as a woman, once had a little girl come up to her in a bathroom and say "Are you really a boy???"
