system
Given the following post, respond as if you're commenting on Reddit:
human
Suppose we (were) visit(ed) (by) an intelligent race of aliens. How could we possibly communicate with them?

I think this is comparable to explorers encountering an indegioness people,  but it couldn't be that simple, could it?
gpt
Richard Feynman covers this in a book.  Basically, start with math, because math should be universal.

Say 'tick one', 'tick tick two', 'tick tick tick three', etc.  They'll probably figure out the pattern if they're intelligent and sense sound.

From there you have building blocks to map mathematical constructs between your language and the aliens' language (one plus two equals three).

Once you have that common ground in place it should be easy to extend it in every direction with a little cleverness.  It may seem trivial but you could teach a lot by extending math into other subjects, like physics and logic, and with those you could turn around and teach things that abstract those concepts, like chemistry and philosophy and so on and so forth up the ladders of abstraction.

You wouldn't so much be teaching each other these concepts so much as teaching words for them, and with that comes a lot of cultural and biological framing for how the other experiences reality.

Some stuff could be hopelessly tricky.  You could explain color with wavelengths but something as simple as 'left' vs 'right' is pretty hard to explain without presence and a common celestial reference, for example.