In operant conditioning, extinction (the cessation of a particular response) occurs when a response no longer results in reinforcement; it occurs in classical conditioning, as discussed earlier, when the CS no longer produces a CR. One way to measure the strength of the learning that has occurred (called response strength) is to see how many unreinforced trials must occur before extinction takes place, (called resistance to extinction). Sometimes, in both classical and operant conditioning, spontaneous recovery occurs; that is, the response in question begins again even though there has been no reinforcement (for example, a rat presses the bar even though the action no longer produces a food pellet).