I'd say that sex is how babies are made. If he pursued it further, I would contintue to answer his questions as simply and directly as I could. But maybe he wouldn't?

 

I had a health teacher who would ask this question, listen to all of our high school attempts at defining/describing sexual intercourse, and then say, "No, sex is male or female". :LOL

So you could always go that route

 

I'm trying to think of the book that Fudge has in one of those Judy Blume books. I want one of those! My older dd just informed my husband and one of her Kindergarten classmates (she's 6) that when she gets older she is going to bleed from her girl parts like her mommy. The kindergarten teacher blew milk out her nose! Then we spoke to 6yo about the appropriate times for saying things LOL. She also told one of her classmates that breasts are for feeding babies. WOW...what I say sinks in, but I'm sure she will offend some mom..

That's okay..some kid keeps telling Marion that she can only go to heaven if she accepts Jesus (we're LDS and almost everyone goes to heaven)..they deserve to hear about breasts and menstration HA.

Sorry for hi-jacking..

I love the advice of all the pp. Keep it light. Four years old is a good age for brief information. At 5 and 6 you can start adding to that. Ask him to say what you told him in his own words. That's what works for me!

 

I'd keep it light, but truthful. Maybe "Sex is something grownups do to make each other feel good." But maybe also try and work into the conversation where he heard about sex and in what context--just in case it was in reference to gender!

 

I'd go with the male/female thing. He's really not asking about sex, he's just asking the meaning of a word he has never heard before. It's no different than if he asked, "Mom, what's a carburetor?" It's an honest answer and isn't one that will trigger additional questions you might not feel comfortable answering right now!