system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do harmonicas come in different keys? Does this mean that to be able to play any song which you like , you need to purchase a set of harmonicas? If no, does a chromatic harmonica capable of playing any song?
gpt
Wind instruments generally rely on perfectly sized holes to create musical notes, and as such the given notes they can produce are fixed.

With large instruments like flutes you have enough 'space' to make holes for lots of notes, so you can create all of the semitones within a given octave. That means all of the little sharps and flats between can be played at any time.

Because harmonicas are small, they economise by using only the notes within a given Key, typically C.

This has meant that the harmonica is easy to play in key, because all of the notes are in key, so it's a simpler instrument to learn to play, and that the Harmonica is a good name for it.

It also means that you get the harmonics of two or three notes playing at any one time, since it's tricky to play just one note on a harmonica.