Language

I think children acquire language by their environment, but also have an innate capacity they're born with to acquire language. I communicate with kids all the time at my job at the museum and it's fascinating the things these kids say to me. Some are quiet and use the wrong tenses and endings. Some use slang, which makes me feel old and confused, and a lot use the same words I would say if I was talking to a friend. These are my favorite conversations. One time I had a boy explain to me all about his weekend and the sushi restaurant he went to. I had a girl tell me all about "High School Musical" and was "shocked" I never saw it. Sometimes we forget that kids are miniature versions of grown men and women. Some are more experienced with language than others, some have learned it from hearing others, some from books. It's interesting to listen to the difference between children in where they have acquired language.
The critical period in young children is a great time to really help kids in learning language and to even teach them a foreign language. I wish I took advantage of that was when I was young. I have always wanted to learn French but now, I think I would have a very tough time grasping it. It's very important to give children that "window of opportunity" for their language(s) to grow, and to give them a positive environment for them to learn from.
When I tutored, all of the children were from non-English speaking homes. It was very interesting in how they communicated with me. I had to help them with using certain words and would ask them to repeat anything they accidentally said in their native language. I would make sure that they knew it was okay to speak their native language but in their school, they need to know English too. A lot of these children got their English from friends, television, and books. For example, one child was a die-hard soccerr fan. He read soccer books and watched it on TV all the time. If I asked him about soccer, and had him write about it, he wouldn't have nearly as much as trouble if I asked him about George Washington, or his Science homework.
The familiarity in daily activities is what helps a child grow with language. But it's also important to open the doors for new words and meanings for them them grasp, so in turn their vocabulary expands as they get older.