阅读理解
Our skin is like a bag that we live in．Inside the bag our bodies are mostly water．Our water is like the water in the sea．It is very salty．Also，like the ocean，we can lose our water．The wind and the sun could take it away．Our bag of skin keeps our body's ocean from drying up．
Our skin keeps out sunshine．Too much sunshine can hurt us．Skin also keeps out dirt．That's important because some kinds of dirt can make us sick．Our skin feels things．It feels warm things，cold things，things it touches，and things that hurt it．A campfire feels warm．A snowball thrown in our face feels cold and hurts us．A hug is the touch of another person's skin on our own．
Our hair is a special kind of covering．It helps us keep things out of our eyes，ears and nose．Hair is also good for keeping us warm．When we get goose bumps （鸡皮疙瘩），our body hairs stand up．Then the hairs hold air close to our skin like a thick blanket （毯子）．Hair keeps animals warm，too．
Some animals have more hair than others，so they have a better blanket for cold weather．
Our nails （指甲） are another special skin which are very hard．They help keep our fingers from
getting hurt．Our nails aren't as strong as the nails that animals have．But they are good for picking up coins．
对任意自然数x，y均满足：
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等于（ ）