In case you really didn't get the joke (or maybe just being sarcastic in acting like you didn't) it was this: When a carbonated drink, such as Pepsi or Coke or Sprite has been left out too long, it looses its carbonation - its fizz. It is then that we call the drink "flat". Of course this has nothing to do with geometric flatness. It is an entirely unrelated meaning of the word flat. But in the OP it was applied to geometric flatness to declare that the world (which is composed of uncarbonated water with no fizz) is "flat".
By the way, for the real nerds among you, this statement is not completely true (that the oceans are not carbonated). Read about
it here. It turns out that the ocean absorbs about 30% of the CO2 released into the atmosphere. Because of the increased used of carbon fuels in the industrial age, the amount of CO2 in the oceans has been going up too, resulting in ocean acidification where the pH of the ocean's surface water has fallen by 0.1 pH units, which represents a 30% increase in acidification because the pH scale is logarithmic, not linear.
Not a pun (need to look up the term for this kind of joke) but this counter joke still fits. (OK I think this type of joke in the OP is called "word play").
