"How can you know there's no god?"

 

"How could the universe have just appeared out of nowhere?"

 

The strawman fallacy is where you debate a position your opponent doesn't actually hold. For example, attacking the position that there is no god would be a strawman against most atheists. Unless they are gnostic atheists, they don't make the additional claim that there is no god. And attacking unrelated positions, such as the creation of the universe, is a common mistake, because that has nothing at all to do with atheism. It is called the strawman because it's like building a fake, easy to beat opponent out of straw and smashing away at it, instead of your actual opponent. But you have achieved nothing if what you are attacking does not line up with what your opponent believes. It's very important to establish this clearly before any debate, to avoid this happening. Some people do this deliberately, so as to create their own line of attack when they cannot find an honest one.