If your preschoolers turn up their noses at carrots or celery, a small reward like a sticker for taking even a taste may help get them to eat previously disliked foods, a UK study said. Though it might seem obvious that a reward could encourage young children to eat their vegetables, the idea is actually controversial, researchers wrote in the American Journal of Clinical Nutrition. That's because some studies have shown that rewards can _ and cause children to lose interest in foods they already liked, said Jane Wardle, a researcher at University College London who worked on the study. Verbal praise, such as "Brilliant! You're a great vegetable taster," did not work as well. The study found t hat when parents gave their small children a sticker each time they took a "tiny taste" of a disliked vegetable, it gradually changed their attitudes. The children were also willing to eat more of the vegetables-either carrots, celery, cucumber, red pepper, cabbage or sugar snap peas-in laboratory taste tests, the study said. Researchers randomly assigned 173 families to one of these groups. In one, parents used stickers to reward their children each time they took a tiny sample of a disliked vegetable. A second group of parents used verbal praise. The third group, where parents used no special vegetable-promoting methods, served as a "control". Parents in the reward groups offered their children a taste of the "target" vegetable every day of 12 days, Soon after, children in the sticker group were giving higher ratings to the vegetables-and were willing to eat more in the research lab, going from an average of 5 grams at the start to about 10 grams after the 12-day experience. The turnaround also seemed to last, with preschoolers in the sticker group still willing to eat more of the once disliked vegetable three months later. Why didn't the verbal praise work? Wardle said the parents' words may have seemed "insincere" to their children. The purpose of writing the passage is  _  .
to introduce a practical method of making children eat vegetables to show the procedure of an experiment on children's diet to explain why children hate to eat vegetables to present a proper way of vernal praise to parents