A wonderful puzzle available at www.alientiles.com. It works on the following simple principle:

You start with a grid of cells. Each cell can be in one of four states, represented by colors: red, green, blue, or purple. In most of the puzzles you begin with all of them red, and the default grid is a 7 by 7 grid.

Each time you click on a cell, that cell's color changes to the next one in the sequence red-green-blue-purple-red...

There are various puzzles to solve which consist of changing from one configuration to another in the fewest moves. Can you turn the whole thing green?