As Shannon said earlier it is a phase that they go through. From birth to about two years old children are in the "Oral Phase" where their pleasure is derived from their mouth. They learn hard, soft, smooth, rough etc etc from their mouth rather than their hands at this stage. Their food is the main source of pleasure for them at this point in time so the putting things in their mouth all the time goes along with that.

The stages continue on until early 20's, with anal, phallic, latent & sexual after the first oral stage. If you want I can post some more info on the other stages as well.......

(BTW this is psychology based info)