题目内容：

The scientists observed that the more junk food the rats ate, the more they wanted to eat – a behavior very similar to that of rats addicted(上了瘾的) to heroin, a dangerous drug. Johnson said the experiment shows that the brain chemistry of obesity(肥胖) and drug addiction may be quite similar.

In their experiment, Johnson and his team studied the “pleasure center” of rats’ brains. The pleasure center is a complicated network of nerve cells. If the animal exercises or eats, the cells reward the animal by releasing chemicals into the body that make it feel good. And when the body feels good, the animal – or person – will want to do the behavior again.

For the experiment, Johnson fed foods like cheesecake to one group of rats. Food like this is high in calories and fat. Another group of rats got a regular diet. The rats that ate junk food started to eat more and more.

“They’re taking in twice the amount of calories as the control rats,” says Paul Kenny, one of Johnson’s colleagues.

Kenny and Johnson wanted to know what was going on in the brains of these rats. They first designed a way to deliver a small electrical charge to the rats’ brains. This electrical charge would stimulate the pleasure centers to release pleasure-causing chemicals. The rats could control how much stimulation – and how much pleasure – they received by running on a wheel. The more the rat ran, the more pleasure it received.

The rats that had been eating junk food started running more and more. This behavior suggested that the junk-food-eating rats needed more brain stimulation to feel good compared with rats on a normal diet. In other words, their pleasure centers were becoming less sensitive and the junk food didn’t make them feel good unless they ate more and more.

Experiments like this one could help scientists understand how chemicals in the brain contribute to obesity. With that information, they may be able to help people avoid obesity in the first place.

(Note: Answer the questions or complete the statements in NO MORE THAN TEN WORDS.)

1.The scientists suggested it was actually ___________________ that made the rats feel good.

2.How did the scientist know what was happening in the brains of the junk-food-eating rats?

3.What was the purpose of the experiment mentioned in the article?