A first grade teacher is teaching a mini-lesson. She has a pocket chart with three columns. At the top of column one are the letters sh; at the top of column two are the letters ch; at the top of column three are the letters th. The teacher has a box with various pictures inside. Examples of the pictures include: ship, thumb, chimp, thimble, shark, and chip. She calls on students to come up, choose a picture, and place it in the proper column on the pocket chart to indicate the initial sound. What is the lesson focus?