human
Suppose a stammerer is engaged in a deep study and unaware of your presence. You speak to him softly. He answers readily, without hesitation, in an absent manner. Again, you ask a careless question, implying by your manner that you do not expect or desire an answer; to this he quickly and easily replies also. Now, look straight at him and pointedly interrogate him. See, when it becomes necessary for him to speak, how he is thrown into confusion.