A teacher showed three toys to his students and asked them to find out the differences. All the three toys seemed to have the same shape, size and material. After looking at them carefully, the students discovered holes in the toys. The first toy had holes in the ears. The second toy had holes in its ear and mouth. The third toy had only one hole in one of its ears. Then the teacher put a needle  in one ear hole of the first toy. The needle came out of the other ear. For the second toy, when the needle was put in its ear , it came out of its mouth. And for the third toy, when the needle was put in, it did not come out. So the teacher explained to his students," The first toy represent those people who seem to be listening to you and caring for you. But they are just pretending to do so. After listening, as the needle comes out from the next ear, the things you said to them are gone. In fact, they don't care for you at all." "The second toy represents those people who listen to you and care for you. But as in the toy, the needle comes out from mouth, these people will use what you tell them to fight against you. They tell others what you said and tell your secrets for their purposes." "As for the third toy, the needle does not come out. It represents those people who will keep the trust you put in them. They are the ones that you can believe." The teacher's purpose of showing the toys was  _  .
to do an experiment to teach toys how to make a friend to give a science class to help students know different people