Rational people think at the margin

Rational people: 

People who systematically and purposefully do the best they can to achieve their objectives.

 Marginal change: A small incremental adjustment to plan of action

• Economists normally assume that people are rational.
• Rational people in life are rarely black and white but the think is they involve the shades of grey.
• Economists use the term marginal change to describe a small incremental adjustment to an existing plan of action.
• Margin and incremental

• Concept of cost benefit analysis
  
  
  
• Marginal decisond making can help explain some other puzzling economic phenomena. Example: why is water so cheap, while diamonds are so expensive? Human need water to survive, while diamonds are unnecessary; but for some reason, people are willing to pay much more for a diamond than for a cup of water.   

      The reason is that a person's willingness to pay for a good is based on the marginal benefit that an extra unit of the good would yield.

A Rational decision maker takes an action if and only if the marginal benefit of the action exceeds the marginal cost