Mostly summarized from Gregory Mankiw’s Principles of Economics, 5th Ed.

PART 1 Introduction
Chapter 1 of 36 Ten Principles of Economics
Section 4 of 14
…
Principle 3 of the Ten Principles of Economics: rational people think at the margin.
Economists assume people are rational, e.g. they want to make money not lose it, they want profits not losses, they want to increase satisfaction not decrease it.
Rational people systematically and purposefully act to achieve their objectives given the available opportunities.
To maximize profits, firms decide how many workers to hire and how much of their product to supply.
To maximize satisfaction, individuals decide how much time to spend working and what goods and services to buy with the resulting income.
…
Rational people know decisions in life are rarely black and white but usually are shades of gray.
At dinnertime your decision is not between fasting or over-eating but whether to take that extra, marginal, spoonful of mashed potatoes.
Your decision for school exams is not between no study or studying 24 hours a day but whether to spend an extra hour of studying.
Economists use the term “marginal changes” to describe small incremental adjustments to an existing plan.
“Margin” means "edge," marginal changes are adjustments around the edges of what you are doing.
Rational people make decisions by comparing marginal benefits with marginal costs.
…
Consider an airline deciding how much to charge passengers who fly standby.
Flying a 200-seat plane across the United States costs the airline $100,000.
The average cost of each seat is $100,000/200 = $500 cost per seat.
One could conclude the airline should never sell a ticket for less than $500.
In fact, a rational airline can often find ways to raise its profits by thinking at the margin.
Consider a plane is about to take off with ten empty seats, and a standby passenger waiting at the gate will pay $300 for a seat.
Should the airline sell the ticket? Yes.
If the plane has empty seats, the cost of adding one more passenger is small.
The average cost of flying a passenger is $500, but the marginal cost is only the cost of a small amount of fuel to carry the extra weight.
As long as the standby passenger pays more than that marginal cost of fuel selling the ticket is profitable.
A rational decision maker takes an action only if the marginal benefit of the action exceeds the marginal cost of the action.
… …
marginal benefit and marginal cost
genkai-teki rieki to genkai-teki hiyō
限界的利益と限界的費用

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