Think about the person who knows you well. Think about that person and all the ways you communicate with him (or her). When you are together, your facial expressions, your sighs, your noises, your body movements - all of them - communicate what you are thinking to the other person. That person who knows you so well can read those non-verbal means of communication and can understand what you are feeling - sometimes even what you are thinking.
If that person, who is not inside your head, who is not with you every moment, can read you like that, then what about God. What about God, whose spirit dwells inside of you, who knows you better than you know yourself?
Why do we think the only way to pray is to form words? Why do we think that we haven't communicated with God?
Could it be that while God is hearing us, we are not listening to God?