Mental development
At this stage of development the mind and the cognitive skills evolve at great speed. They are like sponges and learn each day how to do new things, which means you will be frequently surprised and will be able to follow each of their progresses.
Expressing emotions
4 months:
They roar with laughter, they gurgle, and they even shout to attract attention. If you smile at them, you will see how they smile back. At this age the extrusion reflex disappears, so little by little the instinct to expel through the mouth the objects that enter it goes away and the baby is able to swallow some semi-solid foods, as well as control a little better the sensations of hunger and fullness and will accept or reject the food offered to them.
5 months:
They begin to enjoy games and are absorbed by them, and they start recognising the people who are around them. If you make a noise, you will see how they turn their heads towards it. You will also see how they smile when they see themselves in a mirror.
6 months:
The baby begins babbling. At this time they might also begin to show fear of strangers.