The definition of economics is making tradeoffs in a world of constraints. Examples of constraints include money, time, land, etc. The scarcer something is, the more valuable it is.
Interesting example of a tradeoff: “Bend down and touch the ground, and I’ll give you a penny for doing this.”
Not everyone will say yes to this proposed transaction, and that’s exactly why we find more pennies on the ground than any other unit of currency.