In an elevator, if you stand on a scale, the scale reading will indicate you are heavier when the elevator goes up and lighter when the elevator goes down. Why dos this happen?

     When the elevator goes up,  the elevator floor and the scale is accelerated into your feet, exerting a force that accelerates you upward. By Newton's Third Law of Motion, your feet will exert an equal and opposite froce on the scale you are standing on. This extra force causes the spring in the scale to compress and the reading to increase. The opposite thing happens when the elevator goes down, and so your weight seems to be lighter.