20、Computers can injure you. Most other injuries happen suddenly. For example，if you fall off a bike and break your arms，it happens very quickly. But computer injuries happen slowly.
You probably know how to ride a bike safely. Now learn to use a computer safely.
Your eyes
Too much light can injure your eyes，so never sit too close to a computer screen. Your eyes should be at least 50cm from the screen. Remember to look away from it sometimes. This gives your eyes a rest.
When you use a computer, the window should be on your left or your right. If it is behind you, the light will reflect on (反射) the screen. If the window is in front of you, the sun and the screen will both shine into your eyes.
Your hands and wrists
Hand and wrist injuries can happen because the hands and wrists are moved in the same way hundreds of times. If you use a keyboard for a long time，follow these three rules:1) Rest your wrists on some thing. 2) Keep your elbows (肘) at the same height as the keyboard. 3) Stop sometimes and exercise your hands，wrists and fingers in a different way.
Your back
Some people sit for many hours in front of a computer. If you sit in the wrong way，you can injure your back or your neck. So you should sit with your back straight. The top part of the screen should be in front of your eyes. Your forearms，wrists，hands and the upper part of your legs should all be parallel (平行) to the floor. If you are sitting for a long time，get up every 30 minutes and exercise your arms，legs and neck.
Enjoy your computer，but use it safely.