That awkward moment you catch someone checking you out. So after a few seconds, you check them out to see if they’re still checking you out. And in doing so, they catch you checking them out.
And so you wait a few seconds and then check them out to see if you catch them checking you out again and wonder if they’re wondering what you’re wondering.
Which is:
“Are they checking me out to check me out or are they checking me out to check out if I was checking them out?”