Appeal to mockery, the Horse Laugh, reductio ad ridiculum
Irrelevant Argument, Informal Argument
Presenting the opponent's argument in a way that makes it appear ridiculous
If Einstein's theory of relativity is right, that would mean that when I drive my car it gets shorter and heavier the faster I go. That's crazy! (This is, in fact, true, but the effect is so minuscule a human observer will not notice.)
Point out that this is an attempt to make the argument appear ridiculous. Usually the ridicule will be based on one of the fallacies. Name the fallacy and why it is a fallacy.