Psychologists have many theories to explain how we remember information. The most influential theory is that memory works as a kind of storage system for information. There are three types of these storage systems with different functions that hold information for different amounts of time. They are sensory memory, short-term memory and long-term memory. Sensory memory holds information for the shortest amount of time, less than for seconds and instant. Sensory memory is where stimuli or things that stimulate our senses are very briefly stored. We forget sensory memories almost instantly, unless they passed into another storage system. Examples of the stimuli are what we see and hear in the world, such as a flash of lightning or the sound of a door closing. Short-term memory also called working memory, holds information for about 15 to 20 seconds. This is not a very long time. But the information that passes into this system has more meaning for us than just sensory stimulation. Some experts believe that sensory information changes into visual images as it is stored. And others believe that information changes into words. There is not much room for information in short-term memory. And it did not stay there for very long. Examples of this types of information are telephone numbers, addresses and names. Long-term memory holds information almost indefinitely, although retrieving it can sometimes be difficult. Think of long-term memory as a very big library, with almost unlimited capacities for storage. Information gets filed, cataloged and stored. Long-term memory has several different components or categories. The main two categories are declarative memory and procedure memory. Declarative memory is where we store factual information, such as names, faces, dates, life events. Procedure memory is where we store memory of skills and habits , like how to ride a bike, or how to boil an egg. Within the declarative memory, there are smaller categories of memory, episodic memory and semantic memory. Episodic memory relates to our personal lives. Things we have done or experienced, such as having a car accident or graduating from school. Semantic memory is where we organize general knowledge or facts about the world, such as math formulas, spelling rules and capital cities.