We buy food, which normally are first class products, a supermarket will generally only sell first-class produce, whereas a discount fruit shop might sell second class fruit and vegetables cheaply. This is because the grading system only allows products without any marks or bruising etc to be passed as a first class product. The same principle also applies to food that we prepare or cook. For example if we cook a roast chicken perfectly and rapid cool the item, we have another first class product with a 5 day shelf life if stored correctly. Now we have chicken for taco, fried rice, the kids lunches for the next few days and an array of other dishes. However if we overcook the roast chicken in the first place it now becomes a second class product and will generally be left in the back of the fridge until we clean out the fridge and then toss it. So basically what we need to do when shopping is factor in a few meals that will result in a leftover portion which will be incorporated into another dish in the same week. Another example would be to enjoy a roast on sunday and then enjoy it again as a pie or salad etc.