Why does my baby put everything in his mouth?

Your baby has a natural instinct to put just about everything in his mouth. That because oral exploration is a key part of his development. By putting toys and other times in his mouth, he is discovering tastes and textures of things around him.

Putting items in his mouth may also be a sign that his first tooth is ready to come out. That usually happens around 7 months, but some babies start as early as 3 months. You’ll start to notice that he is drooling more and will constantly want to put things in his mouth.

Being able to rub something against the gum surface can provide some comfort for him, and you might want to ease his pain by rubbing something cold on the gum yourself. The combination of the cold and pressure will decrease the agony he may be going through. Of course, your clean pinky finger might just be all he needs to feel better.
 

Reference:

1. http://www.babycenter.com/404_why-does-my-baby-put-everything-in-her-mouth_6721.bc
2. http://www.babycenter.com/0_teething-remedies-how-to-treat-teething-pain_10357438.bc