1．人脑的重要性及作用。2．如何科学用脑。写作导航：先写大脑的特性和作用(复杂、智能的器官，使人区别于动物等)；再写大脑越用越灵，但过度使用也会出问题，为第三段作铺垫；第三段从两方面说明如何合理用脑：一是劳逸结合，手脑交替；二是从科学的角度，利用数据，说明过度用脑可能给大脑带来的伤害。范文： Human brain is the most complex and intelligent mechanism in the world. It is the major factor that distinguishes man from animals. With our brain we get to know the world and make a good use of the world to our benefit. Our brain is a product of constant use through millions of years. Other things can be used up, but used properly, our brain can never be exhausted. In fact, the more we use it, the more capable and efficient it will become. Excessive use of the brain, however, will causea lot of problems. So it is useful to know how to use our brain wisely. First, handwork or physical labor is good exercise as well as rest for our brain. Doing handwork and brain work alternatively can help develop both our brain and hands. Secondly, we should avoid doing brainwork for long hours on end. The brain, though 3% of the body weight, when at work, consumes 20% of the oxygen carried by 16% of the blood in circulation. A long period of brainwork may cause, through lack of bodily activity, slow blood circulation and inadequate oxygen supply to the brain, resulting in inefficiency and possible damage to the brain.(英语作文)