meaning of sex

Thanks to TOI for this bit of conversation I had to have with my child. Kiddo has started reading smaller words in her school & she is enthusiastic about reading anything & everything. That day being a sunday, TOI's supplementary paper had the article titled 'Sex done'.

Kiddo : Amma, what does sex done mean?
Me : errrrrrrr .... {a long pause &} errrrr ....
Kiddo : 'Done' means as in doing something. But what does sex mean?
Me {after spending considerable time 'thinking'} : Baby, there are two meanings for sex. One says whether you are a boy or a girl. The second .... I don't know how to explain it to you child. Is it okay if we wait till you are 10yrs old. If you school doesn't tell you what it is, I will try to explain it to you.
Kiddo : Okay ... yipppeee!!! {She happily jumps & runs away ...}
Me : Kavilammeeeee .... (My dear Goddess!!!)