You take turns removing 1 or 2 stones from a pile.
Whoever takes the last stone wins!
The trick - if you leave your opponent a multiple of 3 (3, 6, 9, 12, ...), you win for sure.
Example
For parents
What does this exercise practice?
Nim stones practice strategic forward thinking and discovery-based math without a formula - the child finds the rule of three themselves instead of memorizing it.
Common pitfalls
Without the rule of three, the safe move looks like guessing. It helps to think backward from small numbers of stones.
How to practice at home
Play the game with a partner using real pebbles, counters, or chestnuts, and discover the safe remaining numbers along the way.