In most people, laughter can be induced by tickling, a phenomenon in itself. Laughing gas is sometimes used as a painkiller. Other drugs, such as cannabis, can also induce episodes of strong laughter. At rare times, pain can sometimes be used to create laughter though not necessarily with sadistic or masochistic reasoning. Laughter might not be confined to humans. Chimpanzees show laughter-like behavior in response to physical contact, such as wrestling, chasing, or tickling, and rat pups emit short, high frequency, ultrasonic vocalizations during rough and tumble play, and when tickled. Rat pups "laugh" far more than older rats. Chimpanzee laughter is not readily recognizable to humans as such, because it is generated by alternating inhalations and exhalations that sound more like breathing and panting. The differences between chimpanzee and human laughter may be the result of adaptations that have evolved to enable human speech. However, some behavioural psychologists argue that self-awareness of one's situation, or the ability to identify with somebody else's predicament, are prerequisites for laughter, so animals are not really laughing in the same way that we do.