What is perimeter? A perimeter is the line around an object as well as the length of that line. Calculating the length of the line means adding up the lengths of the sides of an object. For four sided polygons, like the ones in this game, one can add the length of each side or multiply opposite sides by two then add the two products together. This geometry game is a fun way to teach perimeter to kids by asking them to select the shape with a perimeter that matches the amount in the question above. Get the question right and the explorer goes up the ladder, getting closer to capture the coin. Get the question wrong and the explorer moves down the ladder and loses some of his heart-life points.