阅读短文，回答问题。

    It's December. It's snowing and the weather is really cold in the north of China. You can see many children playing with snow. Some of them are skating on the river because it's so cold that the water in the river freezes (结冰). It's a white world. How beautiful! But in Australia now, the weather is hot. Many people go swimming in the sea. They are having a good time. Jim and his friends are playing soccer near the sea. The sunny weather is really cool for them. They want to swim after playing soccer.

    Why are they swimming in December? It is winter, isn't it? Yes, it is in China. But it isn't in Australia. That's because we, Chinese and Australians, have different season (季节) in a year.