Babies try to Learn
Babies between four and ten months do not take into account either the hands or eyes to connect with the outside world, do not know how things and people around them.
That’s why they turn to their most direct source of information: the mouth, which is almost the only contact they have to communicate with the world. Need to test the boundaries of such things is to know that they have.
They also need to know if something suck that touch your mouth it is smooth or rough.
They are collecting sensory data through your mouth that are interesting for the baby, and these data are stored in your brain.