Puppies, like babies, use their mouths to investigate new things. This is pretty normal. It's part of how they learn about the world around them. It's also a super fun part of your job as a puppy parent to watch at stop them yay lol.This would be a really good reason to teach both leave, it so that when you see he is interested in something you can stop him. And also drop it ,so if he does have something in his mouth you can get him to spit it out. These are both really important things for a dog to learn and pretty easy to teach.