If we put bits into the mouths of horses so that they obey us, we guide their whole bodies as well.  Look at the ships also: though they are so large and are driven by strong winds, they are guided by a very small rudder wherever the will of the pilot directs.  So also the tongue is a small member, yet it boasts of great things. How great a forest is set ablaze by such a small fire! (ESV)
Imagine a person who seems to have it all. They graduated with a degree from Harvard, first in their class. Perhaps, they have been a part of creating a new cure for a disease. Maybe they have made a lot of money and have even given it all to charity. They seem to be wonderful beyond belief. And then they open their mouth. . .
I have met many people who would have done well to simply keep their mouth shut. You can have all kinds of worldly success, but if you have not learned to control your tongue you are a loose cannon ready to go off and bring destruction at any time. We know many people like this. You might even be one. There is a great expression, "Better to be thought a fool than to open one's mouth and remove all doubt."
James gives three illustrations about how a tiny little tongue has control over our whole body. A bit in the mouth of a horse enables us to guide the whole animal. A small rudder enables us to control the direction of a huge ship. A huge fire is started by one small spark. The tongue is small, but packs a powerful punch.
With such a powerful member of our body we must learn to submit it's use to the Lord's control. Think for just a moment. That tongue can be used to curse our spouse and damage them emotionally. The same tongue can be used to give an encouraging word that can give a drug addict hope. You can heal or destroy. Our tongue has power.
How will you use your tongue today? Will you lift others up or tear them down? Don't try to control the tongue on your own. Ask the Holy Spirit to take control of your tongue.