Although speech is the most advanced form of communication, there are many ways of communicating without using speech. Signals, signs, symbols, and gestures may be found in every known culture. The basic function of a signal is to impinge upon the environment in such a way that it attracts attention, as, for example, the dots and dashes of a telegraph circuit. Symbols are more difficult to disciple than either signals or signs because of their intricate relationship with the receiver's cultural perceptions. In some cultures, applauding in a theatre provides performers with an auditory symbol of approval. Gestures such as waiving and had shaking also communicate certain cultural messages.