I am reading "Joy in the Morning" by P. G. Wodehouse. Around the begining of chapter 3 the following comparison is used: "He spun round with a sort of guilty bound, like an adagio dancer surprised while watering the cat's milk."
It refers to a man suddenly being greeted from behind by a friend while in a jeweller's shop on supposedly delicate buisness.
I don't understand the comparison, which has even been cided, not only in the web, as a particulary witty one. "Adagio dancer" here seems to refer to a kind of acrobat that performs with a partner, one of the two being held in the air in various poses. I have no idea what milk has to do with it. I can't find an expression like "to water the cat's milk" because all I get on google are discussions on whether to give milk to cats. Can someone enlighten me and explain what the comparison means in literal detail? Thank you in advance.