A couple plans on having four children. The father notes that the sample space for the number of girls the couple can have is 0, 1, 2, 3, and 4. He goes on to say that since there are five outcomes in the sample space, and since each child is equally likely to be a boy or girl, all five outcomes must be equally likely. Therefore, the probability of all four children being girls is 1/5. Explain the flaw in his reasoning.