I would use the 'This is a fly in my soup!' just for exclamating that there is REALLY a fly in my soup and pointing it. I can say for myself that I use 'There's a fly in my soup' for joking about the bad quality of the food or to say that something or someone is annoying. For the second sense, I would also use 'There's a stone in my shoe.' In the same sense, I would use the variation 'This is...' just for demonstrating what or who's the fly/stone. Let's see what the natives say about it.

Hi here are the 3 most common versions of this joke; Waiter, what's this fly doing in my soup? Um, looks to me to be backstroke, sir... Waiter, there's a fly in my soup! Be quiet sir, or all the other customers will be wanting one. Waiter, there's a fly in my soup! It's OK, sir, there's no extra charge! All versions of this joke are an attempt by the waiter to stop the customer making a fuss in the restaurant about the quality of the soup that was served, so he makes a joke instead to take the customer's attention away from the soup.

"to find a fly in your soup" can also be used to mean that you found something that was not expected, something wrong, something hidden or sth you hadn´t been told about because you were not suppsosed to notice it.

hahaha.... I only know "a fly in a soup" used for cracking a joke: * Waiter, there's a fly in my soup! It's okay Sir, a while ago it was a cockroach! * Waiter, there's a fly in my soup! Oh c'mon, it can't consume all your soup. (these are some of Philippine version joke about 'the fly in the soup'). but seriously, as we love eating spicy foods (and/or adding some spices to our food), we sometime compare a 'black pepper' to a 'fly' floating in the soup to call someone's attention (especially the attendant) and to open up a friendly conversation. :)

There is no joke. The only jokes are the ones Jura mentioned (and other variations). For fun, see these: http://www.cartoonstock.com/newscartoons/cartoonists/mbk/lowres/mbkn20l.jpg http://www.cartoonstock.com/lowres/ksm0569l.jpg http://www.cartoonstock.com/newscartoons/cartoonists/tzu/lowres/tzun338l.jpg http://ypcommando.com/images/fly_soup.jpg http://rookery2.viary.com/storagev12/718500/718957_8fd0_625x1000.jpg