Final answer:
Lenny has acquired object permanence, the understanding that objects continue to exist even when they are out of sight, which is a significant development in early childhood cognition.
Step-by-step explanation:
Lenny, by searching for a toy that he saw his mother hide first behind her back and then under a pillow, has acquired object permanence. This is a cognitive development milestone where children understand that objects continue to exist even when they cannot be seen, heard, or touched. Preschoolers and toddlers, as they develop this concept, enjoy games like hide and seek and have a better grasp of the permanence of people and objects in their environment.