Womenstruate

I took Katie and Garret to the Dr. today for check ups.  The nurse asked me is Katie had started to menstruate yet.  Both kids heads whipped around because they didn't know what that meant. (I have talked to Katie about it, just haven't used that word.) I shushed them and as soon as the nurse left the room, Garret said "What's menstruate mean?" I told him we would talk about it when we left because I knew the Dr. was coming back any moment.  So before we can even get out of the Drs. office, Garret asks again, "What's menstruate mean?" I asked if we could at least wait to the car to talk about it.  So when we get in the car I explain it to Garret.  He sits and thinks about it for a minute and then asks "so this only happens to women?"  I'm thinking he's going to say something like "I'm sure glad I'm a man" but no his response is "so why do they call it menstruate and not womenstruate?"  Good question Garret.