system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: I went to a dueling piano bar on Saturday night.. How do those guys know all those songs?

The basic set up is two guys playing pianos, you give them $5 with a note of what song you want to hear and they play/sing it.

How do they know so many songs? I get knowing the classics, but it seemed like they were pretty up-to-date on new stuff as well. Not only did they know the piano, they also seemed to know every single word. Like, the actually lyrics, not "every single word" in the way I know the words to songs.

I know that at least one of them had an iPad by their piano, which I'm sure was helping in some way, but even so to play, sing, and read off an ipad at the same time seems insane. If you can do that, you shouldn't be in a bar in Long Beach.

I guess more generally my question is: Is there some trick to this, or are they really that insanely talented?
gpt
The best way to explain this is to say that they take shortcuts. Most pop songs revolve around a fundamental "cadence" or "flow of one chord to another" these cadences are founded in music theory, so if you have a good understanding of music theory you can play the backbone structure of a song in no time. You ever heard the 4 chord song by axis of awesome? 

Go to YouTube and type in "4 chord song axis of awesome" you'll see how it works.