Suppose that meet a Martian. This Martian has spent its entire life drifting in the voids of space, always moving in a straight line (and having a fine old time doing it).
Now, you start talking to the Martian and say something to it like, “My! You should look at those stars over to your left. They’re swell!” (And, let’s be honest, isn’t that the most likely thing you’d say to a drifting Martian?)
The Martian responds by asking, “What do you mean, ‘left’?”
“Oh, you know,” you respond, chuckling, “to your left… on your… left—the side that’s not your right.”
“I don’t understand,” the Martian answers. For a while, you attempt to convey to the Martin just what “left” is, but you find—discouragingly—that you can’t. It’s impossible.