Today I use a short video to introduce the idea of perimeter. I have a farmer who tries to grow lettuce but a very smart bunny keeps eating the lettuce. The farmer is told to build a fence, but he only builds the fence on one side of the lettuce patch. He eventually finds out that he has to build a fence all the way around (the perimeter of his field.)
I show the video and we talk about what the farmer learned. I ask if anyone can tell me what the word perimeter means now that they have seen the farmer? I make sure that students have a grasp of the term before moving to the next part of the lesson.
I have drawn a page shapes document.pdf with a number of different shapes. I ask students to find the perimeter of each shape by measuring in centimeters. We discuss how we might add the 3 or more sides of a figure using an addition strategy that we are familiar with. Students may suggest using tens frames, tens and ones houses, number lines, etc. We reinforce that there are several ways to find the answer.
I have each student work independently and I circulate around to help students who are having difficulty with the process of measuring and then adding more than 2 numbers.
At the end of the lesson I call students to the rug with their perimeter papers. I display one of the figures and ask for a student to tell us what he/she found as the perimeter. I ask other students if they agree. We realize that because we were each measuring and some of the figures were not exactly on the centimeter line, one person might say it was closer to one number, and one to the other. This would change our results just a little. I explain to students that we want to be as careful as possible to start at the zero point of our ruler, but when a line is not exact, we have a choice as to which centimeter we think is the closest. I want students to be precise in their measurements (MP6), but we have not yet worked with quarters of a centimeter so students will move to the nearest full centimeter.
If there is a big difference, I ask for 2 students to tell us the measurements they got for each side and then together we add them up using one of the strategies a student suggests. We may find that a student measured a side in inches by mistake, or that they placed a number incorrectly in the tens or ones place which gave the different result.
Working together we check 2 of the figures and that gives students a sense of how they may have done with the idea of finding perimeter. It also allows for students to correct their own misunderstandings.