季节与活动的关联 — 英语阅读题

Read the passage and answer the question. Hello, I'm Amy. There are four seasons in a year in my city. In spring, it is warm and rainy, so I usually fly a kite in the park with my brother. In summer, it is very hot and sunny, so we often go swimming and eat ice cream. In autumn, it is cool and windy, and my family likes to climb mountains and have a picnic. In winter, it is cold and snowy, so I make a snowman with my friends, but my grandma usually stays at home and drinks hot tea. Question: Why does Amy's grandma stay at home in winter? Answer in English.