Before 8 months, a baby will think that an object has disappeared if it is covered or hidden from view. For example, if a special toy is covered with a cloth, an infant at 7 months will likely lose interest and look away. The concept that objects exist even when they can’t be seen is called “object permanence.” Between 8 and 12 months, babies learn through exploring and testing their environment that objects remain even when covered up or removed from view. At this stage, infants especially enjoy games of peek-a-boo and joyfully removing blankets to discover hidden objects.
Baillargeon, R. (1994). How do infants learn about the physical world? Current Directions in Psychological Science, 3(5), 133
Thelen, E., & Smith, L. (1994). A dynamic systems approach to the development of cognition and action. Cambridge, MA: MIT Press.