In his book The Wealth of Nations, Adam Smith noted the contradiction that, "Nothing is more useful than water: but it will purchase scarce anything; scarce anything can be had in exchange for it. A diamond, on the contrary, has scarce any value in use; but a very great quantity of other goods may frequently be had in exchange for it." This is situation is known as the diamond-water paradox. How does thinking at the margin help to explain the paradox?