Oh… sure! Let's see… [tapping his chin for a few moments]

Okay. A man makes a complaint about the cup of tea he's been served- a fly got in it, because sometimes that just can't be helped. The waiter says oh, that's fine, sir, he'll bring out a fresh cup of tea. So he brings out another cup, but the man objects, saying that this is exactly the same cup as he had before!

How does he know this?