fly in the soup

Teo wrote:

A man asked for a meal in a restaurant. The waiter brought the food and put it on the table. After a moment, the man called the waiter and said: "Waiter! Waiter! There's a fly in my soup!" "Please don't speak so loudly, sir," said the waiter, "or everyone will want one." I cannot understand the above joke. Why will everyone want a fly in the soup?

A variation on the same joke.

Customer:  "Waiter, what's this fly doing in my soup?"

Waiter:   "The backstroke".  [it's a style of swimming.]