incipal that babies are trying to make interesting things last. For example, an infant sucks their thumb, and may find that fun, so it sucks it's thumb more and more. The infant may squeeze a toy, and the toy may squeak. The infant may find that interesting, so it continually squeaks the toy, because it finds it interesting. Also, the infant, at roughly 12 months, begins to lose the train of thought "out of sight, out of mind" and thus start to realize that just because you can't see something doesn't mean it's not there.
             The final development in this stage is that of mental representation. This is the ability to hold an image in their mind for a time after the image was seen or experienced. They can use mental combinations to solve simple problems, such as putting down a toy in order to open a door. They also begin to pretend; tucking a dolly into bed rather then suck on it.
             The preoperational stage lasts from about two to about seven years old. Now that the child has mental representations and is able to pretend, it is a short step to the use of symbols.
             A symbol is a thing that represents something else. For example, a drawing of a cup or saying the word cup is obviously immediately associated by the child with a cup. Thus by thinking with symbols, the child is essentially doing something they could not do in the Sensori-motor stage, think with the absence of the objects themselves.
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