Take a regular ol’ Tic-Tac-Toe board but, rather than filling it with Xs and Os, students will use the numbers 1 through 9.

A few details:

- Each number can only be used once on the board (so once someone places 3, no one can put another 3 on the board).
- Rather than three-in-a-row, players want to add three numbers to equal 15.
- The three numbers can be horizontal, vertical, or diagonal.
- Example: a diagonal of 9 + 1 + 5 would win.
- Non-example: 9 + 6 doesn’t work because you need three numbers.
- Non-example: 4 + 4 + 7 doesn’t work because you can’t use 4 twice.