The learning behavior of human beings can be very complicated. To find out how we learn, scientists often study the activities of animals. Sometimes they observe how long it takes for mice to reach food in the center of a maze( 迷宫) . Sometimes they teach pigeons to obtain food by pressing buttons. In the first experiments in space flight, trained dogs were sent into orbit.They proved that living beings could carry out their activities while whirling about in space. Topic :_________________________________________________________________________ Topic Sentence: __________________________________________________________________