Given scene/plot descriptions from movies or TV shows, and given a specific question about the scene, summarize the question into a title

At the end of Stranger Things' second season, we see that    Will uses Morse code to secretly communicate with Hopper while the monster is possessing him.  Why does Will know Morse code? Did I miss something? Hopper and Eleven know it, Hopper due to his military training, Eleven due to being taught, but Will just doesn't seem to have any particular reason to do so.
Why does Will know Morse code?