Boxing

Gizzmo (white&tabby): I boop you on the head.

Guinness: (black): Don't you dare!

Gzz: Boop!

Gss: Revenge will be mine!

[lots of tumbling, tossing, throwing, squeaking and fur pulling] 

[all of a sudden, they both calm down and wander off in opposite directions, sit for a moment grooming and pondering and wander back to sit near the box]

Gzz: I boop you...

Gss: Revenge!

etc. etc.