Why do babies always stick everything in their mouth? also my son is almost two, is it normal for him to still be doing it?

Comfort/pleasure: The lips are supplied with bountiful nerve endings that make touching or sucking with the lips yield pleasure input to the brain. This helps make sure a baby that has never felt hunger, learns to latch & feed at birth, long before they understand that it makes their stomach full.It is a dominant instinct that persists in adults who chew nails, smoke, etc, all as a self soothing activity.

Of the 5 senses that: Allow us to explore & understand the world, what touches our lips/ mouths in a pleasurable, fulfilling way is one of the first. If that's so rewarding, why not try other things to see if they're as enjoyable? Developmentally, from 6 mos.-3 yrs is typical for mouthing objects. Insuring that access is limited to safe, clean objects too large to swallow or choke on is a parent's responsibility.