If you deliberately use a fake issue to argue against because it's easier to prove wrong than the real issue would be, you're using a straw man. By attacking a straw man, you make it look like you've successfully defeated your opponent's argument.
People use a straw man when they want to appear to have won an argument. A classic straw man might consist of your brother saying, "Cats are great," and you countering with, "Cats aren't better than dogs, because there are no seeing eye cats." Your brother wasn't actually saying that cats are the best animal, but you changed his argument to make it seem that way so you could win the debate.