Economics is about choices since resources are limited. Econ 101 theory tells us that one considers the opportunity cost of making a choice and make a trade off when choosing Good X over Good Y. When purchasing Good X, one considers the relative cost of related goods and makes decisions at the margin. This simple, fundamental, concept is the basis for economics. Economic woman acts in a way that maximizes her utility and considers makes a choice only if the expected value is greater than the marginal cost.
Some mathematician has said that the shortest distance between two points is a straight line. I find that I cannot walk a straight line. Someone moving faster cuts in front of me, then stops without considering the cost they impose on others. If I slow down, someone runs into me. It's always a race to the checkout line at the store. Choices must be made instantaneously. Thus, economics is about behavior that is revealed in every tick of the clock.