Would anyone here (except Dazzleman and htmldon who have already done so) actually try to answer the topic question?
Thanks.
Don't know about you, but at 13, I pretty much already knew the mechanics.
I'd ask them if they wanted the sex talk. I'd tell them that if they have sex without a condom or whore themselves, I'd kill them dead. I'd offer to talk to them about anything they want. I'd tell them that it doesn't matter the hole; if bodily fluids are touching genitals, it's sex. That pretty much covers it.