THE ICE CREAM cone was already invented at a World's Fair, as was iced tea and even, some claim, hot dogs. So what's a guy gonna do to make his fame and fortune at a World's Fair nowadays?

If he is Joe Schoentrup, he's gonna take a pair of scissors and a bag of Fritos and come up with Petro's.

An airline ticket agent in Seattle, Schoentrup started on his path to the fair a year and a half ago in his living room. "I had the idea to go to the World's Fair, but I didn't know what I wanted to sell," he said. After all, he had never done anything in the food field before.

His sister suggested something she had once seen, a bag of corn chips filled with chili and eaten out of the bag. Sounded like a good idea, so Schoentrup called the Frito-Lay company and found that around Dallas such an invention was being sold in Dairy Queens, maybe even in school lunches, under the name chili pie or Frito Pie. Schoentrup liked it, but wanted to make it his own. "We really started adding the gourmet to them," he said, layering the bags of chips and canned chili with shredded cheddar and monterey jack cheese, chopped scallions, chopped tomatoes, sour cream and a dash of hot sauce. He took the idea off to Knoxville and started selling to the construction workers even before the fair opened. By the time the fair began, people were already talking about his hit snack, which had evolved from being called Pepper Bellies to Petro Bellies (Schoentrup thought something sounding like petroleum would better fit the fair's energy theme) to just plain Petro's.

Schoentrup decided to sell Petro's in two sizes, a 3/4-ounce bag of Fritos with fixings weighing in at 9 ounces, for $2.25; and a 2-ounce bag with a finished weight of 17 ounces for $3.75 (which is a good bit more difficult to eat with a flimsy plastic spoon and is a clear example of when more is not better). A plain bag of Fritos was called an Unleaded Petro.

Along the way, Schoentrup developed pioneering techniques for dinner-in-a-bag. "The secret of a good Petro is it's gooshy," Schoentrup discovered. "It's gotta be kind of fluid in there." So his stand stocks canned tomato juice to keep the chili moist in its steamtable pan.

By the second week of the fair, Petro's were selling at the rate of a thousand a day. Schoentrup had already been invited to apply for a stand in the 1984 World's Fair in New Orleans, and someone had offered to buy "the whole Northeast region franchise" of Petro's, which hadn't even begun to expand beyond this one stand run by Schoentrup, his schoolteacher wife and sister and his budget analyst brother-in-law.

A Dallas Dairy Queen sells its chili pies with chopped onions and cheese for $1.30. How had Schoentrup arrived at the $2.25 price for the regular-size Petro's, which cost him 60 cents in food cost? "We just worked it up. We really didn't know where to start at," confessed Schoentrup, who then admitted that the price was high and that maybe he could increase his volume by lowering it.

But after all, Schoentrup was just playing it by ear. He's sort of thinking of having his own bags made, with a Petro's logo on them. Then he could buy corn chips in bulk--Fritos aren't sold in bulk--and fill his own bags cheaper. It's the American way: You start with eating from the bag because it is so convenient, and pretty soon you are making bags just so you can fill them and eat from the bag.