Waiter, waiter, what's this fly doing in my soup?
The breaststroke, I think, sir.
This pun is a play on the verb "doing".
When we say "What's this doing here?" we usually mean "Why is this here? It should not be here." For example: "What's this book doing on my desk? It's not mine."
But when we swim, we can use different styles of swimming like the "crawl", "backstroke" or "breastroke". So we say "to do the crawl" or "to do breastroke". The waiter thought that the fly was swimming.