theres a fly in my stew

mr Grant summoned the waiter and said, "theres a fly in my stew you idiot! i demand a new bowl. Dont try too fool me by removing the fly and returning the same bowl!" The waiter retreated to the kitchen, removed the fly, and returned the same bowl of stew, assuring mr Grant that it was a fresh one. Minutes later an outraged mr Grant called the waiter over and exclaimed, "how dare you try to fool me! this is the same bowl of stew!" how could mr Grant have known?