3. Animal Sudoku
This twist on the classic Sudoku puzzles assigns an animal to each number, ramping up the concentration required to solve each puzzle.

4. Chess With Friends
Multiplayer chess game lets users play against each other or find random opponents. However, it requires a Facebook or Games With Friends login.

5. FairPairHD
Matching game that challenges players to find the associative link (such as rain and an umbrella), and then remember those links going forward as they solve boards with increasing complexity.

6. Finger Physics
Fit together different and moving shapes to solve puzzles. The free version features around 100 levels with various objectives and difficulty levels.

7. Flow Free
Connect matching colors with pipe to create a flow. Pair all colors, and cover the entire board to solve each puzzle. But watch out, pipes will break if they cross or overlap.

8. Genius Brain
Using clues, players must identify in which column a given icon is located on a large board in this version of Einstein’s puzzle. It is aimed at improving your visual memory, ordering your thinking, helping form cause-effect relations, and more.

9. Monorail
Draw lines to connect “stations” and create a working monorail system in this game that challenges visual/spacial/geometric skills. According to the developers, Kindergartners can solve the beginner levels. Mathematicians from MIT have struggled to solve the most difficult ones.

10. iDetective
Players help solve real detective cases using gathered clues, their own logic skills, and a list of suspects. The free version features four different solvable cases.