One person out of every thousand has synesthesia, a psychological phenomenon in which an individual can smell a sound or hear a color. Most of these people are not aware they are synesthetes: they think the way they experience the world is normal. The research field has grown from grapheme-color synesthesia to include other forms of synaesthesia in which flavors are evoked by music or words (lexical-gustatory synesthesia), space structures by time units, colours by music, etc.