A minor third is a musical interval that spans three half steps, or semitones. It is often described as a "sad" or "melancholic" sound, commonly found in various musical genres. For example, the interval can be heard in the opening notes of Beethoven's "Moonlight Sonata," which evokes a somber mood. In terms of piano keys, a minor third can be played by pressing a key and then the key three half steps higher. For instance, if you start on C, the minor third would be E♭. This interval is fundamental in music theory and is used to create chords, such as the C minor chord.