I can't take claim to this original thought, for my mother once came up with it and it has since always made me laugh when it comes to mind. So dogs can't really smile, so they have to show their smiles and happiness through wagging their tails. It's cute and all when they do it- but wouldn't it just be funny if we couldn't smile, and wagged our bottoms instead? Just try it some time, keep a straight face and wiggle your booty when you are happy...
