In captivity chimpanzees can be taught human languages such as American Sign Language (ASL), learning 300 or more signs and can master many complex skills on the computer. It has been demonstrated that chimpanzees are capable of reasoned thought, abstraction, generalisations, symbolic representation and have a concept of self. Although it is difficult to quantify emotions, those who have worked closely with chimpanzees agree that they feel and express emotions such as sadness and happiness, fear and despair – and they know mental as well as physical pain. There are uncanny similarities in the nonverbal communication patterns of chimps and humans – kissing, embracing, patting on the back, touching hands, tickling, swaggering, shaking the first, brandishing sticks, hurling rocks.