Economists are specialised in analyzing trade-offs. At the heart of this field is the notion of scarcity of means (commodities, time) in relation to unlimited needs. This forces subjects to make choices as to how to use their means. Trading means takes place in markets, defined by demand and supply. Market trades occurs constantly, everywhere around the world at a micro and macro level. This yields a dynamic, complex system of interactions that we call ‘the economy’: this is what an economist studies.