If two masses are superposed one on the other and my hand is holding supporting them, the force that is applied on the system is equivalent to their weight. So the reaction of those two masses toward my hand is also equivalent to their combined weight. If mass 1 is on my hand, and mass 2 on mass 1, mass 1 transfers an amount of force to mass 2, which is equivalent to its weight. By action reaction this weight also applies on mass 1. Why is it that the reaction of mass 2 is directly transfered to my hand trought mass 1 and not acting directly on it?