Reaction is a application from 'Cydia' that tests your 'Reflexes' Pretty simple way:

- Once launch application you'll get one big angry face and a cool face at the bottom.
- At a random time, the angry face will turn into a happy face.
- Once you see a happy face, immediately tap on the cool face below.
- A window pop out telling you how many milliseconds you took to tap the cool face after the happy face changed.

If you need more information, jus tap on the 'i' icon at the bottom left of the screen.