The Chooser thinks of a rule that converts one number into another (a mathematician would call this a "function"). The Guesser then tries to guess the rule by testing it with different numbers.

For example, if the Chooser thought of the rule "double it and add one", the game might proceed as follows:

Guesser: "1" - Chooser: "3"

Guesser: "2" - Chooser: "5"

Guesser: "10" - Chooser: "21"

Guesser: "100" - Chooser: "201"

Guesser: "I think it's double and add one" - Chooser: "Correct!"

The rule can be anything you like, but sometimes the Guesser may have to guide the Chooser in the right direction; for example: "Try something larger", or "Try a negative number".

Here are some other suggested rules:

• The number of letters in the word for the number (6 -> 3).
• 100 minus the number squared (9 -> 19).
• The number reversed (17 -> 71)