I agree with Cindi about letting the child determine the common thread after telling her the right name. After all, when a child learns any other word to describe a noun, we don't launch into complicated definitions of what makes it a whatever. Think of "dog" and "cat." Before he was a year old, my son used to refer to all 4-legged animals as dogs. I never explicitly described for him the specific anatomical differences between dogs, cats, cows, etc...but he sure figured it out, and pretty quickly! I'm not one for making a big deal over gender distinctions, either, so that could be part of it. My son still thinks he'll grow up to be a mommy like me, and I figure there are plenty of people who will ruin that one for him later, so I'm not about to do it now!