story
Try something easier, for example two dice. The probability to roll 1 1 is less than to roll 1 2, because 1 2 can be rolled either by first rolling a 1 or a 2, while 1 1 can be rolled only by first rolling a 1. That information is used often in dice games like backgammon.
Same with children: BT BT can only be "rolled" if your first child is BT, whil BT BW can be "rolled" if your first child is either BT or BW.
Do you see?
