When Stanley wants to play with the dog, he will gently bop her nose. Mini wags her tiny tail at Stanley. He bops her head. Mini wags her tiny tail. Another bop. More wagging. Stanley is utterly perplexed by the concept of tails waving in the air in a friendly fashion. Cats who lash their tail from side to side are doing to as a warning, not as a greeting.
Stanley just can't seem to figure out how to get Mini to join in playing with him. When he bops Captain, Captain understand that it's time to play Chase My Brother or Wrestle My Brother. That's when we see two stripey critters careering about the house and up and down the stairs. Mini doesn't do chasing and wrestling. She does cuddling and licking. She appears to think that gentle bops on the head are some form of cat greeting, hence the wagging tail in response.
Crazy critters. :-)