A tic is a sudden, repetitive, stereotyped, nonrhythmic, involuntary movement (motor tic) or sound (phonic tic) that involves discrete groups of muscles. Tics can be invisible to the observer (e.g.; abdominal tensing or toe crunching). Movements of other movement disorders (e.g.; chorea, dystonia, myoclonus) must be distinguished from tics. Other conditions (e.g.; autism, stereotypic movement disorder) also include movements which may be confused with tics. Simple motor tics are typically sudden, brief, meaningless movements, such as eye blinking or shoulder shrugging. Motor tics can be of an endless variety and may include such movements as hand-clapping, neck stretching, mouth movements, head, arm or leg jerks, and facial grimacing. A simple phonic tic can be almost any possible sound or noise, with common vocal tics being throat clearing, coughing, sniffing, or grunting. Complex motor tics are typically more purposeful-appearing and of a longer nature. Examples of complex motor tics are pulling at clothes, touching people and touching objects. Complex phonic tics may fall into various categories, including echolalia (repeating words just spoken by someone else), palilalia (repeating one's own previously spoken words), klazomania (compulsive shouting), lexilalia (repeating words after reading them) and coprolalia (the spontaneous utterance of socially-objectionable or taboo words or phrases). Coprolalia is a highly-publicized, though uncommon, symptom of Tourette syndrome. Typically, tics increase as a result of stress or high energy emotions, which can include negative emotions, such as anxiety, but positive emotions as well, such as excitement or anticipation.