Teka-Teki16.
A brain teaser game for in between. Easy to play at the beginning, but then it becomes continuously trickier.
Sixteen square tiles with two-color patterns are to be joined to form a grid of 4x4 tiles. The only three rules are:
1.) Light to Light
2.) Dark to dark
3.) Light and dark areas must be contiguously (coherent) connected to each other
There are a total of 320 levels to master in six different difficulty stages. Now and then there is a little help from inhabitants of the Teka-Teki16 world.
Once you have mastered all the levels, you can start all over again and continue with new levels. In total, an incredible selection of 24,784 levels is available.