he probably doesn't want or need to know all the details about menstruation yet; like lady_p_gold says, it's just something he's seen and is curious about. There won't be any special mystery about it for him unless you create one, so don't try distracting him, just tell it straight. Just say grown-up women bleed a bit sometime (making it clear you haven't cut yourself or anything, otherwise he'll be worried that you've been hurt), and these mop up the blood like a bandage.