When I was young,I liked to play jokes on people I knew, especially on my parents and friends. One day my mother was cooking and I was playing with my younger brother Tony. Suddenly I ran to my mother and said: "Tony fell from the open window!" She was very worried and ran out of the kitchen. Then I said: "Don't worry, I'm just joking." My mother shouted at me, "If you do it again, I'll hit you." Another day I went swimming with my friends in the sea. I wanted to play a joke on them. In the beginning I swam fast and I called out "Help!". All my friends came to help me. However, they found that I was joking. But half an hour later I wasn't joking. I was so fast, soon I got tired and couldn't swim on in water. I tried my best to call my friends for help, but this time nobody came to help me. In the end they found I was telling the truth. They came and saved my life. They took me to the hospital. This is the best lesson in my life. From then on I haven't joked on anyone. The story tells us that   _  .
swimming is dangerous the writer is a naughty boy one can play jokes only on people he knows if someone always tells lies, others won't trust   him