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A Special Clock Every living thing has what scientists call a biological clock that controls behavior. The biological clock________ plants when to form flowers and when the flowers should open. It tells insects when to __________ the protective cocoon and fly away, and it tells animals and human beings when to eat, sleep and wake. Events outside the plant and animal__________ the actions of some biological clocks . Scientists recently found, for example, that a tiny animal changes the color of its fur because of the __________ of hours of daylight. In the short_________ of winter, its fur becomes white. The fur becomes gray brown in _________ in the longer hours of daylight in summer. Inner signals control other _________ clocks. German scientists found that some kind of internal clock seems to order birds to begin their long migration_________ twice each year. Birds _________ from flying become restless when it is time for the trip, _________ they become calm again when the time of the flight has ended. Scientists say they are beginning to learn which _________ of the brain contain biological clocks. An American researcher, Martin Moorhead, said a small group of cells near the front of the brain__________ to control the timing of some of our actions. These __________ tell a person when to wake, when to _________ and when to seek food. Scientists say there probably are other biological clock cells that __________ other body activities. | 
