In economic terms, these two things are complementary goods.  These are goods that are typically used together.  Because they are used together, the price of one can affect the demand for the other.

Many people like to eat strawberries and cream as a combination.  This is why they are complements.  When the price of a complement goes down, demand for the other complement goes up.  If the price of strawberries goes down, all of a sudden you can afford more strawberries.  When you buy more strawberries, you need more cream to go with them.  Therefore, the decrease in the price of strawberries increases the demand for cream.