Cut a 3 by 4 inch piece of poster board and use the ruler and a pencil to mark it with 1 inch squares. Count the number of squares, remarking that the total number, or area, is equal to the product of the number of rows of squares and columns of squares. Area is the inside measurement of the board.
Arrange the string along the edges of the board and then measure the length of the string. Explain the string is called the perimeter, or the outside. Then measure each side of the board and write down the lengths, noting that the perimeter is equal to the sum of the two long sides and the two short sides.
Cut the board along the lines drawn into twelve boxes, and arrange the boxes into another rectangular shape. Measure the border for that new shape and discuss how this number is different, even though the same number of boxes were used.