During the day we work and play, and at night we sleep. Our bodies rest while we are sleeping. In the morning we are ready to work and play again. When we are asleep, our bodies grow most. When children feel tired and angry, they usually need more sleep. We can get our lessons better, and we feel better, too, when we have enough sleep. Boys and girls, 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 wake up feeling tired. While in bed we must not cover our heads. If we do, our lungs 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 must get plenty of sleep if they want to grow and be strong. How can you get enough fresh air at night? You can   _  .
open your windows not sleep in bed sleep ten hours cover your head