Babies putting things in their mouths is not only normal, but also signifies a burgeoning interest in the world around them.
In their first year, babies are curious and explore their surroundings through their senses -- seeing, touching, hearing, smelling, and tasting. Putting things into their mouths is a byproduct of this development. Baby proofing the house is important to ensure that baby don't put things that can cause him to choke or get sick.
I find that scrunching my face and going "ewwww" or "noooo" whenever small kids put random things in their mouth does help in deterrence to a small extent. Maybe you can consider that?
Anonymous