Bud has solved your problem, and Ill answer the rest of your question. The heater had a dip tube on the cold inlet so that incoming cold water is forced to the bottom of the tank, thus always activating the lower thermostat and element first when your tank is fully hot. Thus the lower element is the workhorse and almost always goes out first. If you use so much hot water that the upper thermostat trips, it cuts off power to the lower thermostat and fires the upper element. That way the water that heats first is closest to the hot water outlet, and furthest from the dip tube. Hope that helps. :
