Psychologist J.P. Guilford, who carried out a long series of systematic psychological studies into the nature of creativity, found that several factors were involved in creative thinking; many of these, as we shall see, relate directly to the cognitive changes that take place during mild manias as well. Fluency of thinking, as defined by Guilford, is made up of several related and empirically derived concepts, measured by specific tasks: word fluency, the ability to produce words each, for example, containing a specific letter or combination of letters; associational fluency, the production of as many synonyms as possible for a given word in a limited amount of time; expressional fluency, the production and rapid juxtaposition of phrases or sentences; and ideational fluency, the ability to produce ideas to fulfill certain requirements in a limited amount of time. In addition to fluency of thinking, Guilford developed two other important concepts for the study of creative thought: spontaneous flexibility, the ability and disposition to produce a great variety of ideas, with freedom to switch from category to category; and adaptive flexibility, the ability to come up with unusual types of solutions to set problems.