Water

There are these two young fish swimming along and they happen to meet
an older fish swimming the other way, who nods at them and says
“Morning, boys. How’s the water?” And the two young fish swim on for a
bit, and then eventually one of them looks over at the other and goes
“What the hell is water?”

This is a standard requirement of US commencement speeches, the
deployment of didactic little parable-ish stories. The story [“thing”] turns
out to be one of the better, less bullshitty conventions of the genre, but if
you’re worried that I plan to present myself here as the wise, older fish
explaining what water is to you younger fish, please don’t be. I am not the
wise old fish. The point of the fish story is merely that the most obvious,
important realities are often the ones that are hardest to see and talk
about.

This is the start of David Foster Wallace’s commencement speech at Kenyon College Class of 2005. I highly recommend it, it is an excellent read.