Suppose that the price of sushi take-out, a substitute, decreases in price. What will happen to the demand for Chinese take-out? The demand for Chinese take-out will likely decrease as a substitute is cheaper. Those we are okay with substituting one item with another are often after the price the items sell out versus exactly what the want from the item. Since they are substitutes, more people will likely get the cheaper option, Sushi take-out.