After introducing the concept of perimeter I had my students measure their math books. Then we figured the perimeter. Next, I had them move to their desk perimeter. They worked in groups of 2 since their desk length/width was more than 12 inches. I called them back together after a few minutes and we recorded their answers. We found the perimeter of the desk. It was 92 inches. The studens (as expected) thought that a mistake had happened in the measurments once I drew a line 92 inches on the board. To prove that we had indead done the measurement correctly, I cut a string of yarn the length of the line. I taped it to a students desk around the edges. I was a perfect fit. This amazed the students and helped them understand perimeter.
Next, I drew an imaginary dog pen that needed to be built. I added an "L" to the side and showed the class how to include this in their figures. We then figured the perimeter. It was 22 feet. WE went outside on the blacktop and laid out 22 rulers to demonstate the lenghth of "fence" that would be needed. Then we used the 22 rulers to construct the pen I had drawn on the board. I explained that if we figured the perimeter incorrectly we would not have enough rulers to complete the design.
My students enjoyed this hands on approach!