Harmony08 makes good points. I heard the mouth described as "the third hand." They are learning that object in every way, building new pathways in their brain.
I have learned with my almost 1YO that after he has investigated an object a couple times, I can sometimes get him to give up the object, using "No" and redirection. I say sometimes, as some objects apparently have much greater appeal that others. That empty toilet paper roll for instance...that is never given up without a fight.