When describing the effectiveness of different types of reinforcers, why is money different from food?

Reinforcement is a psychological term that describes the process of using a stimulus in order to increase or decrease a specific behaviour. The stimulus used is called a reinforcer and it can be classified into several categories. There are primary and conditioned reinforcers. Primary (unconditioned) reinforcers are biologically relevant, vital for survival and occur naturally, without the need for a prior conditioning. Food is an example of a primary reinforcer. On the other hand, secondary or conditioned reinforcers are stimuli which were not originally reinforcing but became such by being paired with other reinforcing stimuli. An example of this is money.