Tics are involuntary, rapid, sudden movements that occur repeatedly in the same way. A tic is a repetitive movement, production of a sound, or even a complex behavior. It is not a "habit". Examples of common vocal tics include grunting and throat clearing. Examples of common motor tics include eye blinking, facial twitching, and head-shaking. Tourette syndrome is a neurobiological disorder with strong genetic components. (Single tics are very common in children, and do not typically require treatment.) For the diagnosis of Tourette syndrome a person needs the presence of multiple (at least two) motor tics and one phonic or vocal tic, which have been present more or less continuously for at least a year. These tics tend to wax and wane over periods of weeks to months. Tics can change, new tics appearing and old tics disappearing, especially in children. The tics tend to get worse with stress or excitement, and to diminish or disappear when the person is concentrating on something or sleeping. The tics themselves are involuntary, however they can sometimes be controlled voluntarily for brief periods of time, but are ultimately irresistible. Tics may also increase when the person is relaxed and in a "safe" environment, and so the tics are often increased when the person is at home. Please note that the almost invisible twitching most people have (at one time or another) around their eyes is not a tic, but occurs in a small bundle of muscle fibres (not an entire set of muscles) and is known as a fasciculation.