Why the First Glass Is Worth More
A glass of water does not have a fixed value to the person drinking it. Its importance depends on what it helps them do. When someone is badly thirsty, water relieves an immediate discomfort and helps the body replace fluid it needs. That first drink can therefore be worth a great deal to them.
Diminishing Marginal Utility
Economists call the extra benefit from one more unit marginal utility. As thirst eases, each additional glass usually adds less benefit than the one before it. The second glass may still be welcome; by the ninth, the person may already have had enough.
This is diminishing marginal utility: the next unit of a good tends to matter less when someone already has more of it. It does not mean later glasses have no value. They may be useful for cooking or for someone else who is thirsty. It means their value to this particular drinker, in this moment, has fallen.
Need Changes What a Good Is Worth
The same logic helps explain why price and usefulness are not identical. Water is essential, yet an extra glass can have little immediate value when thirst is satisfied. In a setting where clean water is scarce, the next available glass can matter far more because it meets a pressing need.
The contrast is a reminder that value is shaped not just by what something is, but by how much of it is available and what a person needs right now.
