"The Stroop Effect is the delay in reaction time between congruent and incongruent stimuli." - Wikipedia
Does seeing the colour red make you read the word red? Test your brain with Stroop!
When the word pops up on the screen, press the colour that the word reads, not the colour that the word is!
Stroop! lets you test your brain to see if you can get the right colour by ignoring the other colours on the screen. 
Now with three free modes:
Classic - play against the clock, how many can you do in 15 seconds?
Focus - how quickly can you get 30 right?
Challenge - take your time, but you've only got one life - don't get one wrong!
Now, unlock the fourth mode with an easy one-time purchase:
Levels mode - challenge yourself with over 100 levels, each harder than the one before.