I came up with this geometry game awhile back after I had this conversation with a 4th grader:

**ME**: What’s perimeter?**4th GRADER**: It’s where when two sides are the same, then you um, I mean you times them by two.**ME**: Ok…I see where you’re going with that. But what does it*mean,*the perimeter of something?**4th GRADER**: I know, but I can’t really explain it.

This girl had learned perimeter as just a random rule she needed to memorize. She didn’t understand that perimeter measures the outside edge of a shape; she just knew how to get the answer.

What she needed was an easy way to understand the *idea *of perimeter, so she’d be able to reason her way through a problem even if she forgets the rule.

Which is why I came up with the Perimeter Game–a super fun geometry game that makes sense of perimeter in a way kids *totally* understand.

### HOW TO PLAY “THE PERIMETER GAME”

**AGE: **7-11 years

**PLAYERS:** 2-4 players

**BEST FOR: **understanding what perimeter means

**PREP: **You’ll need a die and 1 inch tiles. (I have these plastic tiles, but this printable version works just fine.)

**GOAL: **Be the first to make a shape with a perimeter of 24 units

1. Roll the die. **Take that number of tiles**.

2. Use your tiles to **make a shape**. Each tile must touch another tile along a full side.

3. **Count the outside edges** of each tile in your shape. That number is your perimeter.

4. As your shape grows, you can choose to **roll and take more tiles, OR to move one tile** in your shape.

5. The first player to make a shape with a **perimeter of 24 wins. **

### GETTING THE MOST FROM THIS GEOMETRY GAME

Here are some tweaks we made along the way that made our perimeter game even better:

**We kept playing**after the first winner hit 24, to see who got 2nd, 3rd, and 4th place.- I challenged the winner to
**find the area**of her shape (counting total number of squares). - I chatted with the winner(s) while the others played.
**We wondered****if everyone’s shapes would have the same area**. We talked about the difference between area and perimeter. - The girls
**named their shapes**. These girls made a robot and a torch, among other things.

### COMMON MISTAKES TO WATCH OUT FOR

- The trickiest part of perimeter is understanding how to count the units.
**Some girls will want to count each whole side as one unit.** **Many kids count inside corners (“caves”) as one unit**. Make sure they are counting each edge separately.

**It’s easy to lose count**. We found that making a mark on the paper next to each square helped a lot. Some girls wrote the count for each side, then added them all up.

- A lot of kids
**forget where they started**, so keep counting round and round, not sure where to stop. We started making a little mark at the starting place so they knew when they’d made it one time around.

### QUESTIONS AND WONDERINGS ABOUT PERIMETER

I asked the girls some questions as we played…

- What did you notice as you played this game? What else did you notice?
- The robot and the torch both have a perimeter of 24, but the robot used 11 squares and the torch used 15. How can this be?
- How would you change your shape if you wanted a smaller perimeter?
- How would you change your shape if you wanted a larger perimeter?