A Wool Ball.
A Wool Ball is an item for Kitties that can be created by placing four String in a diamond shape in the crafting grid.
Overview
If a Kitty is given a Wool Ball by right-clicking on it, they will play with it for a while by chasing it and pushing it around, until it gets bored. Right-clicking the Kitty again will retrieve the ball.
Crafting
Template:Grid/Crafting Table