The Stroop Effect uses your voice to actually carry out a troop test right on your iPhone or iPad.
A colour appears on the screen and you have to say it: simple. However the word is displayed on the screen in differing modes. Your time to respond is then measured by your device.
John Ridley Stroop studied the effect of of this interference in a widely cited paper in psychology, providing an interesting test for students to measure their reaction time under different conditions.