Next, babies begin to show secondary circular reactions. This sub-stage lasts from about age 4 to 8 months. During this sub-stage, babies begin to repeat actions onto objects outside their body that bring them pleasure and desired outcomes. The difference between this sub-stage and the previous sub-stage is that during this period, babies move beyond just repeating actions to their own body and repeat actions onto their environment. During this time, babies learn by feeling things out; they use their mouths, hands, and other body parts to touch and to experiment with toys and other objects around them. For example, by about age 5 months, babies will track an object with their eyes, even after it leaves their direct line of vision. They will turn their head or even their whole body to continue watching something that grabs their attention. While they're taking in information and practicing cause and effect experiments, their memory continues to grow stronger.
Between ages 8 to 12 months, babies enter the coordination of secondary schemes sub-stage. During this time, they begin to show intentional means-end behavior, which means that babies begin to put different activities together to achieve a goal because they've learned how cause and effect works. Infants are now building on what they learned in the first three stages in order to get what they want. Babies at this age will mimic what they see others doing. If they see their caretaker clap, they will clap. They'll repeat the same sort of experiment with different objects to see how these events are similar or different and if there are different outcomes. For example, they may practice dropping different objects to see what happens. They'll learn that when they stand up and drop a plastic toy on the hardwood floor, it will make a banging noise, but when they drop a stuffed plush animal on the same floor, it will make no real sound.
Another major development during this period is that of object permanence, the understanding that something still exists even if it can't be seen. Before now, babies believed, in an implicit way, that when something moved from their sight, it no longer continued to exist. Now babies begin to understand that something might still exist even if they can't see it. This is how the game "Peek-a-Boo" helps babies learn. Even though they can't see their caretaker's face hidden behind the blanket, their caretaker continues to exist and will reappear shortly.
Next, between the ages of 12 to 18 months, toddlers enter the tertiary (third) circular reactions sub-stage. During this period, toddlers continue to explore their environment and create experiments to see how things work. They will play with anything they can find; however, they do not yet realize that certain things like knives, electric outlets, and pots on top of a hot stove can hurt them. For this reason, parents and caregivers need to be vigilant about keeping their household safe by babyproofing their home.
Object permanence is not achieved all at once, but rather, gradually emerges. During this sub-stage, babies come to realize that something can be hidden and moved and still exist. Now, babies will look for an object that has been hidden or moved. As babies' ability to build memories grows and incorporates all their senses, they develop cross-modal recognition memory. This means that children are able to see a mental picture of an object they are holding in their hand in their mind, without actually looking at it. They remember that object as a complete package through all their senses; they remember its texture and size in their hands, its sound through their ears, and perhaps even its smell.
Finally, between the ages of 18 and 24 months, toddlers enter the beginning of the representational thought sub-stage. During this time, babies begin to be symbol-oriented, which means that they create a general image of things in their minds and retain them as examples of some objects. They may create in their mind a picture of a stuffed bear, and use it to represent other stuffed animals he may play with or later see. Because of this, babies may look for their favorite stuffed animal in the toy basket because they know that's where it's kept even if they didn't see their caregivers put it there. As well, babies' recall and recognition memory also improve greatly. Around age 21 months, toddlers learn scripts, or routines, about how certain things are done. For example, they learn that to "go somewhere in the car," Dad and toddler go out to the garage, Dad buckles baby in the car seat, and then Dad climbs in the front seat and starts the car.
There are other, more specific mental milestones during this period as well. Around age 21 months, babies grasp the idea of past, present, and future. They begin to understand things categorically, which is to say that they become capable of recognize a shirt as a shirt, even though they don't all shirts do not look the same. They begin to recognize what things are alike and why, and what other objects fit or do not fit into particular categories. Toddlers keep building their capacity to think symbolically and categorically Around age 24 months, they develop the capacity to pretend and imagine things that aren't there in front of them. As they achieve this new level of imaginative thought, they take their first steps beyond concrete thinking (e.g., only being able to think about things that are in front of you).