Forgetting is defined as a time-dependent decline of a memory. However, it is not clear whether forgetting reverses the learning process to return the brain to the naive state. Here, using the aversive olfactory learning of pathogenic bacteria in C. elegans , we show that forgetting generates a novel state of the nervous system that is distinct from the naive state or the learned state. A transient exposure to the training condition or training odorants reactivates this novel state to elicit the previously learned behavior. An AMPA receptor and a type II serotonin receptor act in the central neuron of the learning circuit to decrease and increase the speed to reach this novel state, respectively. Together, our study systematically characterizes forgetting and uncovers conserved mechanisms underlying the rate of forgetting.