As children grow up,they become curious about different kinds of things.When they are babies,they are interested in the parts of their bodies and in the smiles of the others.Then they become interested in the physical worldaroundthem:the plants,the animals,and the sky.Later they become interested in the things that people have made: wheels bicycles, cars,etc. And when they are adults,their curiosity continues.Sometimes this curiosity leads to a career(生涯)in science.

　　Scientists spend their lives trying to find out about the world.Those who work with the earth sciences study the earth,the oceans,and the skies.Other scientists who study living things work with the biological science.A third group of scientists study the physical science,e.g.physics and chemistry.

　　These scientists have already discovered a lot about our world.For example,they tell us why your heart beats fast when you run.They say that when you are quiet,your heart normally beats sixtyfive or seventyfive times a minute.Your heart is a pump that pumps blood to all parts of the body.The blood carries oxygen and nutrition.When you run,your muscles work very hard and use the nutrition that the brain carries to them.The muscles need oxygen,too.So your brain sends a signal to the heart.The signal means that the muscles need more nutrition and oxygen.Then the heart beats fast and sends blood quickly to the muscles.It may beat 90 to 140 times a minute.

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