If the title of this post doesn't mean anything to you at first, just say it out loud. It'll come to you. It's a very old joke, and a once-popular vanity license plate, based on the quaint British expression meaning "Does anyone want to come out and get soaked?"
Humans are quite The Brownies' favorite species, and many of the Brownies' antics rise from their attempts to imitate human behaviour and human recreations. Here they have all come out for a game of tennis, but the fact is that they don't know the rules and are making it up as they go along... inevitably, Chaos and Conflict are the result.