If games that challenge your speed and perception intrigue you, then you might want to check out the free Android puzzler 123 Memory: The Numbers Game.
One level at a time, 123 Memory introduces a series of numbered buttons, teaching you the basics with just two at first, before moving on to three, four, five, and so on.
In each level - timed simply by a translucent layer crossing over the screen - the numbers remain only for a split second.
After they disappear, it's up to your own memory to determine which order to press them in.
The mechanics are simple, and the interface even simpler, making for a pretty smooth experience.
You can change the game to portrait or landscape mode in the options menu, and enable "night mode" to make the color scheme easier on the eyes.