I like math puzzles a lot.  0 -zero- comes from Simple Numerical Puzzles.  The object is to slide the row right/left and the column up/down as fast as possible to get a Zero Value in the center.  How many Zeros can you get in the time limit.
This is a good game.  But there should be 3 specific times: 1 minute, 2 minutes & 5 minutes. There should be a Leaderboard feature too.

https://itunes.apple.com/us/app/0-zero/id1375419670"mt=8

...