Riddle Rush is a game in which the player must solve an onslaught of problems in rapid succession. They only take a few seconds to complete, but they test a variety of cognitive skills, and the speed gradually ramps up as the player continues.
Although it is no longer available in the app store, you can click here to play the most recent build of the game. You can select any of the 24 Riddle Rush minigames to play in isolation, or play them all in the main game at varying speeds and difficulties.