Amount of wear, and how long they are may contribute with age and maybe even coloration and pigment.
A sea anemone because they have a symbiotic relationship. The clown fish is kept safe when in the sea anemone from predators. They clown fish also eats the left overs on a sea anemone! :)
Gravitational is only attracted and electromagnetic is both atrractive and repulsive since it depends what you are talking about