the sensor motor stages are the first of four of Piaget's theory of cognitive development. this stage is focused on physical sensations and on developing some basic coordination. children learn by trial and error that they can move their bodies in particular ways, and eventually learn their capability of moving other things. they also learn that people are other objects and acquire basic language.
Object permeance is learned around 8 months of development, meaning they understand that if something is out of sight, it still exists.
For example, if you place a toy under a blanket, the child who has achieved object permanence knows it is there and can actively seek it. At the beginning of this stage, the child behaves as if the toy had simply disappeared.
Piaget observed that babies looking at objects as they were removed from sight throughout the first year, noting that before 8 months they immediately squished attention away from the object once it was out of sight, however from beyond 8 months they continued to look for it. as a result, Piaget believed that that is was from this age that children understood objects continued to exist when they were removed from their view.