Baby jay accidently sucked his thumb, now seems to love to suck his thumb, and has learned to do it over and over again. this best exemplifies the concept of a

primary circular reaction

Explanation:

The situation where a child, after randomly performing an action that causes satisfaction, repeats the same action over and over, is called a circular reaction. During these first months of life, where the object of manipulation is the baby's own body, this behavior is called the primary circular reaction (as when the child sucks his thumb first in a random motion and then repeats this action in view of the satisfaction it generates in the child, similar to what happened with baby Jay).