My mom's explanation when I was little was different in two ways from what anyone has posted:
1. She called it "red stuff" rather than "blood". That's actually pretty accurate, because although it's the blood that has the red color, menstrual flow has several components and is only part blood. It's less scary if you think of it as "red stuff".
2. She said that this is the way an adult woman's uterus cleans itself every so often. This kept me from confusing it with pregnancy or thinking that non-mothers don't menstruate.
I don't think there's anything wrong with the way others have explained it, but I think there are advantages to both of the above.
I remember finding a book about puberty in the church library when I was 7 or 8 and thinking, "Oh, this is that uterus-cleaning thing...so this is how it works!"

No PPAF for me yet, so EnviroBaby hasn't had any opportunity to get curious about it.
