Reaction Time is a simple app that tests your reaction time, like many others in the Google Play store.
The main difference in this app is that it is simple to use and also tests your reaction time in a slightly better manner than most of the other apps.
When you start the app, you are presented with a blue circle that says "Ready". You press and hold the blue circle and release it when it turns green and says "Go:. It's that easy.
The app will keep track of your two fastest times and alerts you when you get a new quickest time. It will also show you your average time after each run. If you want to reset the time, just hit the back button and the next time you run the app the average time will be reset.