There's an old saying： "Laughter is the best medicine，" and modern research certainly shows that laughing is good for you． When you have a good laugh， you exercise your muscles （肌肉），strengthen your heart and balance your body chemistry． However， these are the results of having laugh， not the explanations （解释）of why we do it．

    Until recently， scientists believed that laughter was a human behaviour only． They argued that people find jokes funny because we can put ourselves in another person's shoes， and that animals can't do that．People don't all have the same sense of humour． Sometimes there isn't any joke，or we may not have heard it but we laugh because the people around us are laughing． How long and how loud we would laugh are also shared． We just go on following the example with one another until the laughter fades away （消失）．

    Many pet owners insist that animals have a sense of humour， too． For example， pet dogs often run away with a slipper （拖鞋） and then stop， inviting their owners to run after them． Dogs even seem to smile when their owners join in the game． As the game continues， our pets make noises that are very like laughter． You can compare this behaviour to the laughter of a baby， who continually throws a toy down， waits for the adult to pick it up and throws it down again．

    Perhaps laughter is just a sign that we are having fun． Fun， like laughter， is best when we share it with friends．

（1）The main idea of the first paragraph is　　 of having a laugh

A． the causes

B． the results

C． different ways

（2）Scientists believed that　　．

A． people have the same sense of humour

B． both human and animals can find jokes funny

C． sometimes some people laugh because people around them do so

（3）From the behavior of pet dogs we can tell that　　．

A． pet dogs are very naughty

B． animals have a sense of humor， too

C． animals are more humorous than human beings

（4）When a baby throws a toy down waits for the adult to pick it up and throws it down again， he will be　　．

A． nervous

B． angry

C．happy

（5）The writer encourage us to　　．

A． share fun with friends

B． play games with habits

C． make friends with animals