Instructional methods and teaching methods must be effective so that learning can occur. Direct and indirect instruction are two main categories that many educators find useful for classifying teaching methods, but it is, more complicated than placing all instruction into two categories. Any instructional method a teacher uses has advantages, disadvantages, and requires some preliminary preparation. Often times, a particular teaching method will naturally flow into another, all within the same lesson, and excellent teachers have developed the skills to make the process seamless to the students. Which instructional method is right for a particular lesson depends on many things, and among them are the age and developmental level of the students, what the students already know, and what they need to know to succeed with the lesson, the subject-matter content, the objective of the lesson, the available people, time, space and material resources, and the physical setting. Another, more difficult problem is to select an instructional method that best fits one's particular teaching style and the lesson-situation. There is no one "right" method for teaching a particular lesson, but there are some criteria that pertain to each that can help a teacher make the best decision possible.