The practical significance of this factor is, of course, great. When ideas are presented to pupils these ideas should be made clear. Every conceivable device should be used to clarify and explain,--concrete demonstration, the use of objects and diagrams, pictures and drawings, and abundant oral illustration. We must be sure that the one taught understands, that the ideas become focal in consciousness and take hold of the individual. This is the main factor in what is known as "interest." An interesting thing is one that takes hold of us and possesses us so that we cannot get away from it. Such experiences are vivid and have rich emotional connections or accompaniments. Ideas that are experienced together at such times are strongly connected. =Mental Set or Attitude.= Another influence always operative in determining the association of ideas is mental set. By mental set we mean the mood or attitude one is in,--whether one is sad or glad, well or ill, fresh or fatigued, etc. What one has just been thinking about, what one has just been doing, are always factors that determine the direction of association. One often notices the effects of mental set in reading newspapers. If one's mind has been deeply occupied with some subject and one then starts to read a newspaper, one may actually miscall many of the words in the article he is reading; the words are made to fit in with what is in his mind. For example, if one is all wrought up over a wedding, many words beginning with "w" and having about the same length as the word "wedding," will be read as "wedding." Mental set may be permanent or temporary. By permanent we mean the strong tendencies that are built up by continued thought in a certain direction. One becomes a Methodist, a Democrat, a conservative, a radical, a pessimist, an optimist, etc., by continuity of similar experiences and similar reactions to these experiences. Germans, French, Irish, Italians, Chinese, have characteristic sets or ways of reacting to typical situations that may be called racial. These prejudicial ways of reacting may be called racial sets or attitudes. Religious, political, and social prejudices may all be called sets or attitudes. Temporary sets or attitudes are leanings and prejudices that are due to temporary states of mind. The fact that one has headache, or indigestion, or is in a hurry, or is angry, or is hungry, or is emotionally excited over something will, for the time, be a factor in determining the direction of association.