We all stumble in many ways. Anyone who is never at fault in what they say is perfect, able to keep their whole body in check.
When we put bits into the mouths of horses to make them obey us, we can turn the whole animal.
Or take ships as an example. Although they are so large and are driven by strong winds, they are steered by a very small rudder wherever the pilot wants to go.
Likewise, the tongue is a small part of the body, but it makes great boasts. Consider what a great forest is set on fire by a small spark.