On a pleasant fall or spring day, take your class outside to calculate the perimeter of the school and playground, using themselves as measuring devices. Assign one or two students to tabulate the results. Line the children, shoulder to shoulder, along one side of the building or playground. When everyone is in position, ask them to count off, from left to right, starting with one. The tabulators should record the total. The first person in line then moves to stand next to the last person. The rest of the line uncoils, like a snake, to form a new line next to the leader. Count off again and record. Continue this way around the school or playground. When a complete circuit has been made, the tabulators determine, using addition or multiplication, how many third-graders it takes to measure the perimeter.
Divide the class into groups of four. Provide large pieces of poster board or butcher's paper, crayons, markers and pencils. Instruct students to illustrate examples of situations in which it is necessary to determine the perimeter of something -- for example, framing a picture, laying carpet, planning a garden or building a house. Display and discuss the finished murals. As a followup assignment, ask them to measure the perimeter of something at home, using whatever measuring tool they wish and share the results with the class.
Prepare worksheets listing parts of the body, such as the arm, wrist, leg, hand, head and waist. Cut string into lengths from 6 to 24 inches. Students should use the pieces of string to measure the perimeter of each body part on the list. To make the activity easier, allow children to work in pairs, assisting each other with holding the string in place and using thumbs and forefingers to mark the beginning and end of the measurement. Lay string along a ruler or yardstick to discover the perimeter dimension. Record the results on the worksheet.
Create cardboard cutouts of squares, triangles, rectangles, octagons and other multi-sided figures. If "polygon" is a new word for your class, define and spend some time discussing examples. For this lesson, children work in pairs, one to measure and the other to note results. Give each pair a set of shapes and a measuring implement. The task is to find the perimeter of each polygon. When finished, tell the students to exchange the shapes and repeat the exercise. Proceed with a class discussion covering any disagreements in results and the possible reasons for them.