Final answer:
When the price of a product changes, it causes both a substitution effect and an income effect, which leads to changes in the quantity demanded.
Step-by-step explanation:
When the price of a product changes, it causes both a substitution effect and an income effect. The substitution effect occurs when a good becomes more expensive and consumers seek out substitutes. For example, if the price of oranges increases, people may buy more apples or grapefruit instead. The income effect refers to how a change in price affects the purchasing power of the consumer. When the price decreases of a good the consumer can purchase the same quantity of goods as before and still have money left over to buy more. Both effects lead to changes in the quantity demanded.