Fly, Soup, The Usual

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.”

Why will everyone want a fly in the soup?