If you want your kids to have great eyesight, send them outside to play each day.
New UK research shows children who play outdoors halve their chance of needing glasses for short-sightedness.
Researchers at Bristol and Cardiff universities found children who spend a lot of time outdoors were almost 50 percent less likely to be short-sighted when they were 15 compared with those who remained inside as children.
The researchers asked the parents of 14,000 eight- and nine-year-olds how much time their child spends outdoors.
Kids who spent more than three hours outdoors in summer and more than an hour outdoors in winter were classified as "high". Any less was classified as "low".
The results showed time spent outdoors was a more important preventer of myopia (short sightedness) than being physically active — but they are still unsure why this is the case.
The researchers hypothesised that daily exposure to bright light could stimulate the neurotransmitter dopamine in the retina.
Jeremy Guggenheim, a reader in optometry and vision science at Cardiff University, who contributed to the research, said this study shows that the long-held belief that bookworms have eye problems is probably not true.
"There doesn't seem to be much connection between the time spent on different indoor activities, such as reading, and myopia," he said.
"The more people have tried to nail down reading as the cause, the less convincing the evidence has looked."
The study is published in the journal Investigative Opthalmology and Visual Science.