The sun sends a lot of different rays to Earth. You can see some of them as sunlight. Some of them you can't see. The ones you can't see are called "ultraviolet" rays or "UV rays." These rays are the ones that can make your skin burn.
From the sun, UV rays travel through space until they reach the ozone layer. It stops most UV rays from getting any closer to the Earth. But as the ozone layer gets thinner more UV rays get through. We need some UV, but too much of it can be a dangerous thing. It can harm people and wildlife, such as frogs.
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