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.

为了探究滑动摩擦力大小跟哪些因素有关，小明同学在老师的指导下，做了一系列实验，以下是部分实验步骤（实验过程中，保持长木板水平）．

第1次：把木块平放在长木板上，用弹簧测力计水平拉动木块，（图甲），读出弹簧测力计的示数，并记录下表中．

第2次：把木块侧放在长木板上，用同样的方法拉木块，（图乙），记下相应的示数．  

第3次：把两块相同木块叠在一起平放在长木板上，再用同样的方法拉木块，（图丙），记下相应的示数：