Reinforcement is anything that increases the likelihood of a response occurring again. There are four main types of reinforcement: primary, secondary, positive, and negative. Primary reinforcement satisfies biological needs, like food or water, while secondary reinforcement becomes rewarding through association with primary rewards, like money. Positive reinforcement increases behaviors by rewarding them, like praising a child for doing homework. Negative reinforcement reduces behaviors by removing an unpleasant stimulus when a behavior occurs, like studying to reduce worry about a test.