My seven yr old wants to know what sex is! Heeeellllpp!

[TheDreadedLurker]

So putting him to bed tonight he just comes out with,"What is sex?"
Just totally caught off guard (yes, I really should have prepped for this in advance but I'm just thinking seven is so LITTLE!!)
Initial response was to ask who had told him that word so it turns out the class know-it-all kid is telling all and sundry that it is "something dirty".
Not a great place to start from, I'm thinking!
I really would have loved to have preserved his innocence/ignorance for a couple more years :-( So what should I tell him?

buy him a book all about it. we got dd1 a book from the family planning association Let's talk about where babies come from and she dips into it from time to time. it's great - sex in context of loving relationship, all about pregnancy and birth, contraception, STI's - even genes and chromosomes!

Thanks everyone!
Some great support here and thus I was SLIGHTLY more prepared this morning at breakfast when the question was repeated!
To try and dispel the whole "it´s a dirty thing" idea from his charming (ahem) little friend, I said:" You know when you see people kiss on TV and you say, oooo gross! ?- well it´s a bit like that, but its not gross at all, it´s a special thing grown ups do when they love each other"
He seemed very satisfied with that and next question was, "Can I have another bowl of Weetabix?" so think that will suffice for now!!
If he asks more specific Qs I will not lie to him, but I feel no need to go into a whole lot of detail...PLENTY of time for that later!
I went for the "people who love each other" angle as I think it´s way simpler, as some posters have stated, than trying to explain lust, attraction, sex drive, need for affirmation, dominance...... fill in any further reasons for wanting sex here
Will just need to keep a sharp ear open fo any further inaccuracies coming from precocious playmates