Money & WorkMental model
Why the third slice of pizza is worth less than the first.
Marginal utility is the extra benefit from the next unit of a good, and it almost always falls. The first glass of water when thirsty is priceless; the fifth is a nuisance. Likewise, going from €0 to €1,000 in savings adds far more security than from €10,000 to €11,000. It's why an extra euro helps the poor more.
Useful: Before buying more of the same, ask whether the next euro would do more good elsewhere.
Source: de.wikipedia.org