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Here is a standard Yin-Yang puzzle.

Rules of Yin-Yang:

  1. Fill each empty cell with either a black circle or a white circle.
  2. All white circles should be orthogonally connected, so do all black circles.
  3. There may not be any 2x2 cell region consisting of the same circle color.

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I couldn't find a suitable starting logic without a not-well-known trick:

In a puzzle with even width and height, the border cells excluding the corners are divided into dominoes, each of which is to be colored in a single color. So the bottom left corner starts as this:

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Apply some easy deduction and use the "border pair" logic once more:

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Some more easy deduction, and we realize that the white must go all around the board by border connectivity logic. Sneaky!

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Then the chain of easy deduction finishes the puzzle:

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    $\begingroup$ "Fill each empty cell with either a black circle or a white circle." ;-) $\endgroup$
    – Florian F
    Commented Nov 29, 2020 at 15:14
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    $\begingroup$ @Florian I believe that has been left as an exercise for the reader. $\endgroup$
    – Daniel Mathias
    Commented Nov 29, 2020 at 15:59
  • $\begingroup$ Correct and well done! :D (The technique is apparently mentioned way back then here in PSE: A problem about Yin-Yang puzzles) $\endgroup$
    – athin
    Commented Nov 30, 2020 at 0:57
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    $\begingroup$ @athin Yeah, I learned it through that exact post :) $\endgroup$
    – Bubbler
    Commented Nov 30, 2020 at 1:00

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