The communication chain is the process which occurs inside your brain when you're having a conversation with someone.

It begins at Look and moves clockwise.  You look to someone when they speak and hear their words. You interpret non-verbal communication such as an angry body stance, or tears and you concentrate on what you're hearing.

The words stick inside your brain (remember) and then you begin to process them first by understanding the words, then the sentences and finally the meaning.  

Once this process is complete you can begin to answer.

First you have to had an idea, decide how to articulate and then find the words to match those ideas. The sentences need to be put into a sentence and you say the words. You also adapt mid-speech to the other person as you begin to pick up on the visual cues that they are giving you.

Whilst this all seems pretty straight forward, bear in mind that this happens every time you you hear something and respond. Within a conversation that could be HUNDREDS of times and literally hundreds of thousands of times per day. If a child has any gap at any stage within this chain, their chances of communication are affected. 

Whilst we accept children are capable of communicating complex sentences, it is important to still give them at least ten seconds to process this information and reply. This time will decrease as they get older or if you ask them less complex sentences.