What is purchasing power parity? Why might it not hold?

What will be an ideal response?

Purchasing power parity is the idea that similar foreign and domestic goods, or baskets of goods, should have the same price when priced in terms of the same currency. Purchasing power parity may not hold because countries produce different baskets of goods and services, because some goods and services aren't traded internationally, and because transportation and legal barriers may prevent prices of traded goods from being equalized.