Nim
Take stones from one pile. Take the last to win.
How to play Nim
Normal-play Nim starts with piles of 3, 5 and 7 stones. Each turn removes one or more stones from exactly one pile. Taking the last stone wins.
Tap a stone to remove it and every stone to its right in that pile. Hard computer play uses optimal Nim strategy.
Example: 3, 5, 7 → take from one pile