Why do we laugh? For years scientists have asked themselves this question. No other animals laugh and smile, only human beings. So does laughter help us to survive in any way? The answer seems to be that smiling and laughter help us to relax, reducing tension and preventing illnesses which are caused by stress. Laughter really is the best medicine! But if you're unhappy, it can't do you any good to smile, can it? Not true. Apparently, the positive effects of smiling are just the same whether you feel like smiling or not. Scientists in California asked a group of actors to act five different feelings; happiness, sadness, anger, surprise and fear—just using their faces. Changes in their heart rate, blood pressure and temperature were written down. Most of the feelings they acted didn't cause changes, but when the actors smiled, their heart rate and their blood pressure went down. This happened to make them feel better. A sense of humor also helps to keep things in perspective. Interfering friends or a flat tire won't seem quite so bad if you can see the funny side. So if you find yourself in a stressful situation, try to imagine how it must seem to an outsider with a sense of humor and let yourself laugh. A psychologist writes, ' If people try to show a positive reaction instead of a negative one, this will eventually become part of their personality. If you start to laugh, in time you will actually become happier. How do people differ from other animals?