You may hear someone say, "It's raining cats and dogs."We know it means it's raining hard. Here is how it comes.
    There are many large rats in a country. People need cats. But there are many mountains in the country. It's hard for a plane to land (着陆）． So the cats come down by parachute（降落伞）．
    In another country, the people need dogs to find people in the snow. Where can they get the dogs? You can guess. The dogs also come down by parachute.
    These cats and dogs are helping the people. And the people often say it rains cats and dogs.