“Once You See It, You Can’t Unsee It

“The original photographer is unknown, but this particular shot is from Facebook.”
I love optical illusions, especially ones that really twist your brain around. I saw one recently that really had me going for a minute. And it’s not so much the illusion itself that really gets me, but my own brain’s reaction to it.
The photo is above. I saw it on a Facebook post from this week, though it’s been around since at least 2014.* It shows a brick wall, seen at a shallow angle, with somewhat large gaps between the bricks. The bricks are red, and it appears that there’s a small gray rock stuck in between them just above center.
So what’s the illusion? I couldn’t see it at all, even after a good 30 seconds of staring at it. I was starting to suspect there was no illusion, and it’s a gag to fool people, when I read the comments and realized what I was missing.
SPOILER ALERT
If you still haven’t seen it, then what follows below will spoil it for you. If you don’t want to know then don’t read any further until you’ve figured out the illusion!

This illusion nags at me because with the duck-rabbit illusion my brain easily switches back and forth… I see the duck, I see the rabbit, I see the duck, I see the rabbit… try it yourself.

But with the illusion in question, once I see it, I can’t unsee it. What’s going on? What are the implications? There must be some. Perhaps many. I’m intrigued. So is Twitter.

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