Professor Ian Morgan，from the Australian Research Council’s Vision Centre， said，“Humans  are 【小题1】 long-sighted，but when people begin to go to school and spend little or no time outdoors, the number of short-sighted people gets larger. We’re also seeing more and more short-sighted children in cities all around the world and the main reason may be that city children spend 【小题2】 time outdoors. ”

Daylight can be hundreds of times brighter than indoor light. But why does playing outside prevent us from becoming short-sighted? Scientists believe that natural light has a special chemical（化学物质） which stops the eyeball from growing out of shape and 【小题3】 people from becoming short-sighted.

So be outdoors. It doesn't matter 【小题4】 that time is spent having a picnic or having sports.