On Friday evening, Sam McPherson had a change in plans. He was going to pick up the twins from the music school, for them to spend the weekend at home with the family. Earlier on this day, Sam gave his class a test. Four students flunked the test, and he blamed himself for that unfortunate mishap. So, the professor did what he felt was the right thing to do. He scheduled another session with the four students in the evening. First, he did a better job reviewing the subject matter. Then he gave them the test. While the students worked on it, he called the twins to tell them that he will pick them up early on Saturday. He tried to call them twice, but there was no answer. He finally dropped it. He went to the hallway to drink from the water fountain. When he returned, he sat by the desk, and
