comment, in a substantive manner to the following post

1) Why do indifference curves have a negative slope?

Indifference curves represent a curve where combinations of bundles of goods and services represent the same level of utility? For two goods, this is easy to see graphically. If good Y is plotted on, let's say, the vertical axis, and good X is plotted on the X axis, you can see the relationship easily. Any point (X,Y) would be a bundle. Several bundles graphed together, assuming they provide the same level of utility (benefit) would result in a curve. This is negatively sloped because as you decrease utility from the amount of one good, you must increase the amount the other good to maintain the same level of utility, and vice-versa. This creates a negative slope.

2) What is a Giffen good?

A Giffen good is a good is a good that can theoretically defy the Law of Demand. If the good is inferior, the income effect can possibly dominate the substitution effect and cause the slope of the demand curve to be positive. This would be a low income, non-luxury item where demand rises when price rises and demand falls when price falls. Obviously contrary to the Law of Demand. Certainly rare.