It's early Saturday morning, you're lying in bed, and you realize that there's no food in your house. You've got to go to the grocery store. You've also got to visit your elderly parents and take your teenage children to soccer practice that afternoon, so you have limited time for grocery shopping. You decide to do the grocery shopping right away because, although you'll be able to spend less time lying in bed, the really good grocery deals sell out by 10 AM on Saturdays, so in the long run, you'll be able to buy more food if you get out of bed now. And, because you spent all but $30 of your cash last night playing bingo and you've maxed out your credit card, there's a limited amount of food that you can buy; you'll need to get as much food as you can per dollar. It needs to be enough to provide dinner for your children and a few of your nephews and nieces who will be coming over after soccer practice, absolutely starving. Although $30 probably won't buy enough food to stuff all of them, it will buy enough so that your children and their cousins won't die before morning.
So you drive to the grocery store and race down the aisles. You, your children, and their cousins eat all kinds of things, so choosing what to buy is more complicated than if, for example, you were koala bears eating only eucalyptus leaves. And you can't just buy what you and your children like best (caviar and lobster) because $30 won't buy enough of that. You also can't buy just foods or just drinks; consuming food without drink, or vice versa, isn't at all appealing. You have to figure out the right combination. In addition to these problems, you wish that you could remember which aisle the soda is in so that you wouldn't have to spend your precious time looking for it. While you're looking for the soda, it occurs to you that maybe you should just spend the whole $30 on food for yourself. After all, you're