When one cat is playing with something, another cat may come along and see what fun the first cat is having. Notice Orca, the black and white cat, in the picture below. She's perched on top of the tub, looking down at Merlin who is playing with a plastic ball with a bell inside.
She thinks "that looks like fun - perhaps I will join him".
She jumps into the tub, and when the ball rolls near to her, she bats it away. Merlin and Orca take turns batting at the ball with their paws, making it jingle and chasing it around. For them, this is fun.
Now allow me to show you how dogs react in a similar situation.
Zara finds a plastic bag. What fun!
Ziggy spots Zara with the bag, and thinks "That looks like fun - perhaps I will take it!"
Which becomes a wrestling match.
Where the bag is all but forgotten.