Guthrie's contiguity principle offers practical suggestions for how to break habits. One application of the threshold method involves the time young children spend on academic activities. Young children have short attention spans, so the length of time they can sustain work on one activity is limited. Most activities are scheduled to last no longer than 30 to 40 minutes. However, at the start of the school year, attention spans quickly wane and behavior. problems often result to apply Guthrie's theory, a teacher might, at the start of the year, limit activities to 15 to 20 minutes. Over the next few weeks the teacher could gradually increase the time students spend working on a single activity. The threshold method also can be applied to teaching printing and handwriting. When children first learn to form. letters, their movements are awkward and they lack fine motor coordination. The distances between lines on a page are purposely wide so children can fit the letters into the space. If paper with narrow lines is initially introduced, students' letters would spill over the borders and students might become frustrated. Once students can form. letters within the larger borders, they can use paper with smaller borders to help them refine their skills. The fatigue method can be applied when disciplining disruptive students who build paper airplanes and sail them across the room. The teacher can remove the students from the classroom, give them a large stack of paper, and tell them to start making paper airplanes. After the students have made several airplanes, the activity should lose its attraction and paper will become a cue for not building airplanes. Some students continually race around the gym when they first enter their physical education class. To employ the fatigue method, the teacher might decide to have these students continue to run a few more laps after the class has begun. The incompatible response method can be used with students who talk and misbehave in the media center. Reading is incompatible with talking. The media center teacher might ask the students to find interesting books and read them while in the center. Assuming that the students find the books enjoyable, the media center will, over time, become a cue for selecting and reading books rather than for talking with other students. In a social studies class some students regularly fall asleep. The teacher realized that using the board and overhead projector while lecturing was very boring. Soon the teacher began to incorporate other elements into each lesson, such as experiments, videotapes, and debates, in an attempt to involve students and raise their interest in the course. The purpose of this passage is to ______.