During the day we work and play; at night we sleep. Our bodies rest while we sleep. In the morning we are ready to work and play again. While we are asleep, our bodies grow most. Children who are tired usually need more sleep. We can get our lessons better and we feel better too when we have a good rest. Boys and girls who are eight or nine years old need ten hours of sleep every night. Our bodies need plenty of air when we sleep. If we do not get enough fresh air we'll feel tired when we wake up. While in bed we must not cover our heads. If we do, our lungs（肺）will not get enough fresh air. If we open our windows at night we can have plenty of fresh air. Cool air is better than warm air. Boys and girls who want to grow and be strong must get plenty of sleep.

象棋中的楚河汉界源自项羽和刘备之间的战争。