I can understand it if the string has some sort of object attached, because this vaguely resembles a mouse and might trigger stalking and pouncing instincts. A ball of yarn
would fit this pattern. But my cats seem to love any string-like object—a spool of ribbon, the plastic twine that sometimes comes with the newspaper, etc.
String-like objects look like a mouse tail or lizard tail? (We live in Florida, so lizards are high on the 'enemy-alert-must-be-chased list.
(Remember, anything that is not nailed down is a cat toy. HyperKitty’s favorite is a belt loop from an old pair of jeans.)
You’re not kidding about anything not nailed down. One of my cats, who seems to have a slightly more literary bent, actually goes out of his way to play with pens. I don’t know what the attraction is, other than the string-lizard-mousetail complex you mentioned. But I suspect Rocco is fascinated by how if he pushes one end of
the pen, the other end moves in the opposite direction.
Not having passed high school physics, he doesn’t understand the principles of levers and fulcra. So to him it’s endlessly fascinating.