A harmonica is a small musical instrument you play by blowing air through it and drawing air back in through your mouth. It fits in your pocket and consists of a rectangular case with holes along one side. Inside each hole sits a thin metal reed that vibrates to create sound when air passes over it.

The harmonica produces different notes depending on which hole you blow into or draw from. By moving it back and forth across your lips, you can play melodies, and skilled players can even play chords by covering multiple holes at once. Some people call it a mouth organ or harp for short.

Invented in Germany in the 1820s, the harmonica became wildly popular because it was affordable, portable, and relatively easy to learn. Cowboys carried harmonicas on cattle drives, soldiers played them in wartime, and blues musicians made the instrument famous for its soulful, expressive sound. You might recognize its sound from folk songs, blues music, or movie soundtracks. Bob Dylan often played the harmonica while singing, using a special holder that let him play both guitar and harmonica at the same time.

The harmonica remains popular today partly because you can learn simple songs quickly, yet master players spend years developing techniques like bending notes to create that signature bluesy wail.