Video narration
Arin walks the playground's outside edge and returns to the start. That walk is perimeter: distance around a closed figure. A frame, table, or playground has an outside edge to follow. A closed figure has a complete boundary; an open line stops short, with no complete route around it. For a polygon, add every side length once. This triangle has sides four, five, and seven centimetres: four plus five plus seven equals sixteen centimetres. A six-centimetre by two-centimetre rectangle and a square with four-centimetre sides look different, but each boundary totals sixteen centimetres. Perimeter measures the distance around, not the space inside. Which have complete boundaries: the triangle, open line, or rectangle? [Brief pause.] The triangle and rectangle. Now try six, eight, and five centimetres: six plus eight plus five equals nineteen centimetres. Remember Arin's walk: perimeter is the full distance around a closed figure, counted along every side.