In his attempt to explain the difference between Importance and Expression to Grendel, the dragon uses the examples of the difference between vegetables and animals and the difference between an angry man and a rock. Explain.

The dragon uses the examples of animals and vegetables to explain about man's inevitable move towards complexity. Vegetables can be regrown by using a bit of its part, while an animal has a head that controls every other part. Likewise, man has the ability to adapt to any change in his environment, which leads to the choices he make. And it is this superior ability of man which signifies the gap between man and other beings, just like importance and expression are different.

Explanation:

When Grendel went to the cave of the dragon in Chapter 5 of the text Grendel by John Gardner, the dragon began teaching about everything. He proclaimed that he knows and hears everything, exclaiming that it's what "makes [him] so  sick and old and tired".

And in his attempt to teach Grendel about the difference between importance and expression, he uses the example of a jug. He states "the jug is an absolute democracy of atoms", but devoid of expression. To him, "Importance passes from the world as one to the world  as many, whereas expression is the gift from the world as many to the  world as one." This shows that what is deemed important may not necessarily be expressive.

In other words, his attempt to explain the difference led to the animal/vegetables analogy. He explains nature's consequent move to complexity which he relates to the case of man's ability to adapt to his environment despite Grendel's belief of their foolishness. He uses vegetables and animals as examples between man and a rock. A rock is a simple object, which may be whole in its being but unlike a man, it is not capable of doing anything to change its existence. On the contrary, man can adapt to his surroundings and change his ideas, even at the last minute. Animals have a head which leads the other body parts, like a man does. But vegetables and rocks have no such ability, which renders their limited abilities. To summarize is points, the below quote from the text rightly presents the difference-

"If the  dominant activity be severed from the rest of the body--if, for  example, we cut off the head--the whole coordination collapses, and  the animal dies. Whereas in the case of the vegetable, the democracy  can be subdivided into minor democracies which easily survive without  much apparent loss of functional expression."