This was a project I did for my Arduino prototyping class in which we grabbed an unlabeled random sensor out of a box and had to create a project around it. I received a microphone sensor and, after determining that the sensor was not sensitive enough to react reliably to quick bursts of sound (like snapping, clapping), but did respond to blowing directly on the sensor, I made a candle simulator. 

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The sensor sits at the base of a line of 4 LED lights, set to "extinguish" one after the other as the user blows harder on the microphone (registering as a louder sound). If the user makes it all the way to the 4th candle (an RGB LED), it turns green, and they win!