Of Names and Surnames

“What is the surname of an auto?”, my older one piped up suddenly from the back seat of the car the other day.
“Autos don’t have surnames”, I wanted to reply, but something made me say, “I don’t know. You tell me.”
“Rickshaw!”, he announced triumphantly.

I am not sure exactly when and how he learnt about surnames, but the five-year old is obsessed with them now.
“Mamma, what is my surname?”
“Talwar.”
“What is my brother’s surname?”
“Talwar.”
“What is Papa’s surname?”
“Talwar.”
“What is your surname?”
“Ramarathnam.”
“But why is your surname different?”
I knew he was not going to understand if I tried to explain why I chose not to change my surname after marriage, specially since the class teacher seemed to have drilled it into her kids that all members of a family have the same surname. I could tell him that my Papa and Mamma were my family, and that I shared my surname with them, but that would probably have led him to ask if I was not a member of his family.
Time for half-truths. “Because I am a girl. Girls have different surnames.”
This should buy a bit of time, but I wonder if the next time he asks the same question he would be ready for a more accurate answer.
Is that why women change their surnames– because it is just so much simpler. Or have I made a tactical mistake by not giving both my kids double barreled surnames?