Passage 3 (79) There is an English saying: 'Laughter is best medicine,' Until recently, few people took the saying seriously. Now, however, doctors have begun to look into laughter and the effects it has on the human body. They have found that laughter really can improve people's health. Tests were carried out to study the effects of laughter on the body. People watched funny films while doctors checked their heart, blood pressure, breathing and muscles. It was found that laughter has similar effects to physical exercise. It increases blood pressure, the heart heating and breathing; it also works several groups of muscles in the face, the stomach, and even the feet. If laughter exercises the body, it must be beneficial. (80) Other tests have shown that laughter appears to be able to reduce the effect of pain on the body. In one experiment doctors produced pain in groups of students who listened to different radio pro- grams. The group that tolerated the pain for the longest time was the groups which listened to a funny program. The reason why laughter can reduce pain seems to be that it helps to produce a kind of chemicals in the brain which diminish both stress and pain. As a result of these discoveries, some doctors in the United States now hold laughter clinics, in which they help to improve their patients' condition by encouraging them to laugh. They have found that even if their patients do not really feel like laughing, making them smile is enough to produce beneficial effects similar to those caused by laughter. 11. Doctors have proved the following EXCEPT that ______.