Don't Say 21

How to Play

1. Players count upward starting from 1.
2. On each turn, say one, two or three consecutive numbers.
3. The next player continues from the following number.
4. The player who is forced to say the target number — 21 by default — loses.
5. Use the controls to change the target and explore how the winning pattern changes.

Find the Control Numbers

Instead of thinking forward from 1, reason backward from the losing target. Ask which number you would like to hand to your opponent so that every possible reply can be answered. With a maximum of three numbers per turn, the strategic spacing between control positions is four.

Strategy Tips

• Work backward before the game starts and identify the positions you want to reach at the end of your turns.
• If the opponent says one number, you can often answer with three; if they say two, answer with two; if they say three, answer with one.
• Changing the target is a useful way to test whether you understand the pattern rather than memorizing one sequence.

Game-Theory Connection

This is a small impartial sequential game closely related to subtraction games. It demonstrates backward induction, winning and losing states, strategy invariants and how an apparently conversational game can have a precise mathematical solution.