The Three Types of People :

A teacher shows three toys to a student and asks the student to find out the differences. All the three toys are seemed to be identical in their shape, size, and material. After keen observation, the student observes holes in the toys. 1st toy it has holes in the ears. The 2nd toy has holes in ear and mouth. The 3rd toy has only one hole in one ear.
Then with the help of the needle, the student puts the needle in the ear hole of the 1st toy. The needle comes out from the other ear. In the 2nd toy, when the needle was put in the ear the needle came out of a mouth. And in the 3rd toy, when the needle was put in, the needle did not come out.