Children in kindergarten learn about six shapes. circle, square, triabgle, rectangle,diamond and oval. I learned that the most effective way to introduce a shape is to start with a straight line. I would show students a wire covered with chenillle. They used to be called pipe cleaners. I don't know what they are called now but you can get them at a craft store and are about 12 inches long. I would then show the students how the shape was formed I would do it with each shape introduced (e.g.so they could see the differences between a square and a diamond, a difference that even escaped our principal when she came to observe.) We would look for the shape around the room. (e.g. square - window, rectangle - door etc.)
We would also make a book the same shape as each shape as the shape just introduced. (e.g. a round book shaped like a circle.) The first page would say trace a circle. the second page said color a circle, the third - trace a circle, the fourth, draw a circle. The fifth page said, Cut a circle and included a small piece of paper with a circle drawn on it. Students would cut it out and glue it to the page. The last page was labeled, "This is a Circle Picture." Students would make a picture using the introduced shape. (e.g. a circle shaped cookie, triangle Christmas tree, rectangle robot, etc.) We learned a song for each shape and continued to revue them throughout the year. Interesingly students never had trouble identifying the required shapes.
What I learned: Repetition is sometimes necessary for remembering. The key is making the repetition interesting.