hi emx, my cat who lived to the ripe old age of 20 ( sadly we lost her a few years ago) retrieved those little orange plastic arrowns you get in toy guns. We found this out by accident when my grandad bought me them as a kid. She did it from young right to old age and she really, really loved it - when we said 'where's y'arrow?' she'd go mad with excitement, looking for it under the sofa so that we'd throw it for her. It was a real novelty for us and visitors to see this - try to associate it with a word or phrase and she may end up doing it for life like my cat! I like to think it's because she was clever :-)
Love to kitty
x