eli5: why do we know a note is off key even when we’ve never heard the piece of music before?

Question

Whenever I hear someone play an instrument, I can always tell when they’ve played the wrong note even if it’s my first time hearing it, why is that?

In: Other

Answer 1

Music is written in keys. Only 8 notes go in a key (usually, I think three are exceptions.) so if you play a note not in that key, it is obviously wrong.