Place a large piece of felt on a bulletin board. Make a square in the middle with strips of felt, leaving a border. Make a classification set with several different attributes. Something that works well is to make a set using shapes, colors, and varying sizes. For this example you might make five large triangles that have different colors, and five small triangles with these same colors. Do the same for several other shapes. You can decide if you want the board to be a one-difference train or two-difference train. For a one-difference train, the student would start with any shape. If they choose a large red triangle, for the one-difference train they might place a large red square next to it. For a two-difference train, they might place a small red circle next to it. Students will continue with the train until they have used all of the pieces. You can add challenge to this bulletin board by telling the students to connect the ends of the train together to complete the sequence.