This is Water.

Two young fish are swimming along when they happen to meet an older fish swimming the other way. He nods at them and says:

"Morning, boys. How’s the water?"

The two young fish swim on for a bit. Eventually, one of them looks over at the other and goes:

"What the hell is water?"

This little anecdote comes from David Foster Wallace’s famous 2005 commencement speech, "This Is Water."

See transcript here, and audio recording here

What is the water I am not seeing?

What is the thing so constant I’ve stopped noticing it?