In my stat mech book i seem to have a problem with the answer to this question which seems really straight forward; Q/ Mr and Mrs X have 2 children. If they tell you that at least one of them is a boy, but you dont know the gender of the other child, what is the probability that they have two boys? A/ 1 in 3. There are four 'microstates': - BB BG GB and GG. By telling you that at least one of the children is a boy Mr and Mrs X have only eliminated the GG microstate. The point is that there are two microstates (BG and GB) in the 'macrostate' of having a boy and a girl. Children are distinguishable particles My problem with this, is that since we are told that at least one of the children is a boy which leaves only the possibility that the other child is either a boy or a girl, hence a 1 in 2 chance. Why does the order BG or GB matter in such a scenario?