Women simply do not have to complete our sentences when we are having a conversation. There are many versions of this (as there are dialects in the english language).
Example One... My sister and I can be talking about our kids and I will be sharing my abundance of wisdom in child rearing with her and she will complete all of my sentences for me... out loud... with enthusiasm... to my dear husbands dismay. (Not that he doesn't enjoy her company, it's just that she speaks very loudly.) I will of course let her do this as it makes the conversation proceed much quicker. Quicker in the sense that we have more opportunity to repeat ourselves as we women love to do! Repetition, it's good for... (annoying men) memorization, getting your point across, (annoying men) applying what you've learned .
Example Two... My dear friend Sophie and I will be sitting at the dinner table with our blessed husbands sharing what has been going on in our lives. I will be telling a story, or making a point about something, and will simply not complete my sentence/thought. (Picture 2 woman continuing on with their meal and 2 men holding their next bite in their hand, wide-eyed, mouth agape waiting for the climatic conclusion.) At this my husband will break the silence with a question, "Could you please finish what you were saying?" I will simply look at him as if he just asked me what color the sky was (duh) and say, "Sophie knows what I meant/was saying." That is because we women tend to repeat ourselves, so we can just recall a previous conversation similar to this one and complete the thought. Therefore, we can finish each others' sentences! Brilliant of us! (My husband and Sophie's husband don't think so.)
Example Three... If you are a woman reading this, you have already thought of your own. If you are a man, my apologies...