Prepare a handout with the cipher or give each group a set of letters for them to figure it out themselves. For example, A = 1, B = 2, C = 3, .... Z = 26. It is recommended to have on hand a premade list of the codes: 14 15 --> NO, 2 15 15 11 --> BOOK, etc.
1. Split the students into groups.
Playing by rows: First person in each row will write on the board while the second person breaks the code and yells out the corresponding letter.
Playing in lunch groups: Each group can be given a whiteboard or they can raise their hand to answer.
*Can be modified into a pair or an individual activity. "
2. Reward points to the groups who break the code. It's recommended to reward the fastest two or three teams, instead of only one as this prevents any one team from dominating the game.