Its been a decade since I flew 121 but I'm coming back, help me understand how you use these coolers. lets say you have a three day trip. Do you really bring 3 days worth of food and keep it cold for 3 days? if so, how do you keep it cold all the time, seems like it would be a job itself to keep on top of it. I totally understand for a day trip. I'm sure I'm missing something. If its non perishable stuff you bring why does the bag need to be a cooler?