They absorb everything around them. Everything we say, everything we do. It is incredible. We do not always realize it until they repeat the word or mimic the act. But they watch and hear it all. Both the good and the bad.
We are all guilty of it. We say something without thinking then realize our child has just heard it. Obviously it cannot be taken back but we can try our best not to repeat that word or sentence. Eventually they will hear it and say it, but we try our best to shelter them until they are old enough to understand it. In today's information age, however, that understanding can come much earlier than when we were children. Protecting our children from that vast amount of readily available information is the topic for another (or many other) posts.
As much as they absorb the bad, they absorb the good. Funny noises. How to laugh. How to talk. Our son, for example, has learned to give kisses and enjoys doing so. I am not sure exactly who taught him, but he likes to grab my face with his hands and give me a kiss. It is cute when he does it, but not something I expected him to do. He obviously learned it from someone as that is not something he would just start doing I don't believe.
We are happy for him to learn new things, sometimes they make you wonder...