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A Biological Clock Every living thing has what scientists call a biological clock that controls behavior. The biological clock tells _____1___ when to form flowers and when the flowers should open1. It tells _____2____ when to leave the protective cocoon茧 and fly away, and it tells animals and human beings when to eat, sleep and wake. Events outside the plant and animal _____3_____ the actions of some biological clocks. Scientists recently found, for example, that a tiny animal changes the color of its fur ____4_____ the number of hours of daylight. In the short ____5___ of winter, its fur becomes white. The fur becomes gray brown in color in the longer hours of daylight summer. Inner signals control other biological clocks. German scientists found that some kind of internal clock seems to order birds to begin their long migration _____6____ twice each year. Birds _____7_____ flying become restless when it is time for the trip, ____8___ they become calm again when the time of the flight has ended. Scientists say they are beginning to learn which ____9____ of the brain contain biological clocks. An American researcher, Martin Moorhead, said a small group of cells near the front of the brain ____10____ to control the timing of some of our actions. These ____11____ tell a person when to _____12_____, when to sleep and when to seek food. Scientists say there probably are other biological clock cells that control other body activities. | 
