Does a spring pour forth from the same opening both fresh and salt water?  Can a fig tree, my brothers, bear olives, or a grapevine produce figs? Neither can a salt pond yield fresh water. (ESV)
There are certain things about nature that we should be very grateful for. I am thankful that every single day I can count on the law of gravity. It helps to know that I can walk across the floor without worrying about whether my feet will hit the ground with each step. No worries about floating through the air. The law of gravity is a constant.
James uses some illustrations from nature that remind us of how we are to be constant with the words that come out of our mouths. When people see us open our mouths they should have confidence that they will hear words that are full of blessing, not cursing. When there is inconsistency in what comes out of our mouths it causes great confusion because it does not match our claim to be a Christian.
We do not get fresh and salt water from the same source. We do not pick olives from fig trees. We do not get figs from grapevines. You can make up more scenarios like what James has done. The point is simple. We are to produce fruit that is consistent with a regenerated Christian. Consider the fruit that should come from our lives. We find a list of things that are fruit in a Christian in Galatians:
 But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, (ESV)
This fruit should consistently come from our lives. In fact, it will often express itself through what we say. If we are full of love, then loving words will come from our mouth. If we are full of joy, we will say words that express that joy, and so on. We are new creations in Christ Jesus. As we draw close to him he will bear his fruit in our lives and we will begin to express what is now inside us in the way that we use our mouth.
Do you want to live a Christian life that does not confuse others? Draw close to Christ. Allow him to bear his fruit in you. Out of the outflow of what is in you the fruit of the Spirit will be expressed in what you say.