For years people have watched snake charmers in Indian bazaars play music to their cobras. The watchers have marveled at the way the snakes seem to move in response to the music. But a little research shows that a snake cannot hear sounds that are high pitched. If a cobra is blindfolded, you can play music on a pipe for hours and the snake will sit still. If you remove the blindfold and wave your arms around, the snake will raise its head and spread its hood. A cobra responds not to the music, but the movements the snake makes as he plays. It is difficult to enter the world of animals. Too often we think of animals as if they were human beings. It is hard to realize that they do not see as we see, smell as we smell, or hear as we hear. Yet anyone who does not realize this is bound to make mistakes. Many experiments can be done in the field, outside of the laboratory. If you change some factor in the life of a group of animals, how will the animals respond? And how does this behavior compare with the behavior of the control group kept in normal surroundings? Scientists have to be very careful to make sure that the animals are reacting only to the sights and sounds provided in the experiment and not to something else. Even the experimenter can ruin an experiment. One scientist was studying how geese responded to a cardboard model of an enemy. The enemy was a bird of prey flying overhead. Before each test, the experimenter had to climb a tree to fasten the model overhead. Very soon the geese were giving an alarm call every time the experimenter started to climb the tree.