The idea is simple. You’ve got a bunch of bolts and a pile of nuts stacked on top of them in all the wrong colors. Your job is to fix the mess.
Tap a bolt, and you pick up the top nut. Tap another, and you drop it. Easy, except you can’t just put a red nut on top of a green one. Same colors only, or into an empty bolt. That’s the whole rule.
It starts off relaxing. A couple of bolts, two or three colors, no big deal. Then more colors show up, fewer empty spaces, and suddenly you’re stuck because you used your last open bolt too soon. You’ll backtrack, try again, and when it finally clicks, when every bolt is stacked neatly in one color, it’s a little victory that feels better than you expect.
There’s no timer, no pressure, just you versus the puzzle. And that’s what makes it addictive. One more level, one more stack, until you realize you’ve been sorting nuts for half an hour.