Daydreams are a form of imagination. In daydreams, the person forms a mental image of a past experience or of a situation that he or she has never actually experienced. The daydreamer may use these mental pictures to escape from reality temporarily, to overcome a frustrating situation, or to satisfy hidden wishes. Although most psychologists view daydreams as generally healthy and natural, this was not always the case. In the 1960s, for example, textbooks used for training teachers provided strategies for combating daydreaming. Daydreams occur in frequencies set by biological cycles of temperature and hormone levels, or they can be triggered by a sensory input such as sight, taste, smell, sound, and touch. Psychologists estimate that the average person daydreams about every 90 minutes, and daydreams peak around the lunch hour (noon to 2 p.m.). Daydreaming first occurs for most people during childhood, sometime before the age of three, and these early daydreams set the pattern for adult daydreaming. Children who have positive, happy daydreams of success and achievement usually continue these types of mental images into adulthood. Daydreams become the incentive for problem-solving, creativity, or accomplishment. On the other hand, children whose daydreams are negative or scary are more likely to experience anxiety and fear, and this pattern will carry over into adulthood as well. A child's daydreams may take a visible or public form - the daydreamer talks about his or her mental images while he or she is experiencing them and may even act out the scenario he or she is imagining. After the age of ten, however, the process of internalising daydreaming begins - the child no longer expresses but continues to form them.