When you see two babies while they are playing, you will notice that they do understand each other pretty much. Like there is some kind of universal language which allows them to communicate with pretty unusual selection of gestures and voices.
The following video which has became incredibly popular all over the world is showing two small baby-twins while they are having an argue about something. They use “dadada” words and it seems that they perfectly understand each other and they even look like that they are expressing more complex thoughts and feelings. This conversation doesn’t look so simple at all and it seems that they are pretty enjoying while having it. At one point I was started to think that this is definitely going to end with a fight but it seems that they were just having a fun. If you are a linguist then this little conversation could be very useful material in studying the nature of language and communication.