© David Kaplan.
The strong force is strong because when particles with color charge are pulled apart, the force strength does not diminish, as it does in QED. The force field is said to form a string or tube between a quark and anti-quark being pulled apart. When the string gets long enough, it contains more energy than it takes to create new particles by converting that energy into mass. In that case, a quark and anti-quark pair can appear out of the vacuum, canceling the color charge of each of the other two, and breaking the string.