Reductio ad absurdum

In formal logic, reductio ad absurdum (Latin: "reduction to the absurd") is an argument to refute a proposition (or set of propositions), by showing that it leads to a logically absurd consequence. That is, the proposition is shown by proper inspection to be simply untenable within the rules of logic, because it necessarily leads to a self-contradictory consequence.
(Reference: http://en.wikipedia.org/wiki/Reductio_ad_absurdum)

Quote from the TV Show the Big Bang Theory:
Leonard: "Penny if you promise not to chew the flesh off of our bones then you can stay"
Sheldon: "He's engaging in reductio ad absurdum, its the logical fallacy of extending someone's argument to ridiculous proportions and then critisizing the result"