This is the classic perimeter and area lesson based on a farmer's field. With various manipulatives or drawing on graph paper, have the students figure out how much fence Farmer Bob needs to enclose a field that holds a particular area of crop. Change the area, or change the required perimeter and let the students figure out if the area changes along with it. For added fun, create the field (on a smaller scale) outside using stakes in the ground and string for the fence.