In Charlotte's Web, how did the old sheep get Templeton to say that he would bring another word back to Charlotte for her web?

At first, Templeton refuses to go the fair, where he could help Charlotte by running errands, doing "general work"—and finding a word for her web.
This attitude changes when the old sheep appeals to Templeton's self interest. He tells the rat that a fair is a rat's paradise because it is filled with all sorts of food that people spill. It contains the remains of peanut butter sandwiches, bit of cheese, popcorn, pieces of donuts, caramel apples, and many other foods that appeal to rats. When Templeton gets interested and asks if this is really true, the old sheep adds that the food pickings at the fair will more than surpass Templeton's wildest dreams. He describes sour mash on the sides of buckets, empty tuna fish cans, and more. He tells Templeton that:

a fair has enough disgusting leftover food to satisfy a whole army of rats.

This so entices Templeton's appetites that he decides to go. True to the character of the rat, he goes not to help save Wilbur but out of his own self-interest.