Primary vs Secondary Reinforcement
Primary reinforcers is a biological reinforcer. Basically, this means that the primary reinforcement is something that the animal innately needs (food, water, sleep, etc.)
Secondary reinforcers are reinforces that have no inherent value. The reason a secondary reinforcer is rewarding is because it has some sort of link to a primary reinforcement. So for example, the click of a clicker can be used as a secondary reinforce. Since the click is associated with food, the sound itself becomes a reward in its own right. This links back to classical conditioning, where a neutral stimulus is linked with a stimulus that naturally causes a certain reaction (think Pavlov's dogs and the bell. The bell initially meant nothing, but by ringing it before giving food (which naturally causes a drooling response), the bell was linked to the response towards the food, namely drooling.)
Food is the best reinforcer for all animals, including rats. So when training a new trick, its a good idea to use food to reward the rat for doing what you're asking for.
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