Zotz! is an enjoyable time-waster, a form of perceptual practice, good for training the brain and the eyes; there is no timer and no score, no pressure, just a simple challenge waiting for you whenever you like.
The game supposes a deck of cards, where each card has four attributes, and each attribute on any given card has one of three possible values: Number (one, two, or three), Color (red, blue, or green), Fill (solid, hazy, or empty), and Shape (diamond, oval, or rectangle).
All you have to do is to spot, amongst the visible cards, any three cards which, for each attribute, are either all identical or all different.
Colors, middle fill type, and number of cards in the layout can all be customized. Full instructions, with examples, are included.
Updated for iOS 9 and iOS 10; rewritten in Swift 3. Minor bug fixes: improve restoration of layout in practice mode, reduce likelihood of getting stuck in a 3x3 game.