The demand for a product is influenced by the price of related products. The professor is discussing that the demand for a product is influenced by the price of related products. There are two kinds of related products: substitute goods and compliment goods. Substitute goods are products that are similar enough to be replaced by each other. For example, button and margarine can be substituted for each other, when the price of the button is increased and become less affordable, people would buy margarine instead, and then demand of the margarine will go up. Compliment goods are products that compliment or complete each other. For example, CD and CD player are compliment products, and they can’t be used without each other. When the price of CD goes up, there will be fewer people to buy it, and then the demand of CD player will decrease.