Some scientists think that one day animals may be trained to do a number of simple jobs that are now done by human beings.
Perhaps you have seen some animals doing quite skillful things at a circus. If you watch closely, you may notice that the trainer always gives the animal a piece of candy or a piece of fruit as a reward. The scientists say that many different animals may be trained to do a number of jobs if they know they will get a reward for doing them.
Of course, as we know, dogs can be used to guard a house, and soldiers both old and modern times have used geese to give warning by making a lot of noise when a stranger or an enemy comes near. But it may be possible to train animals to work in factories. Birds with good eyesight are being used to watch out for fault in small steel balls that are being made in one factory. Apes have been used in America in helping to make cars and scientists believe that these large monkeys may be trained one day to gather crops and even drive trains.

What made scientists think that animals can be trained to for us one day ?( )