I went into this weekend with high hopes of finishing the pet mat for my neighbor’s dog Oreo, but while I made substantial progress, finish it, I did not.
Yesterday, at some point in the midafternoon, I finished crocheting both of the large crochet circles I would need for Oreo’s pet mat:

While I was mostly happy with how the two crochet cookies had turned out (having managed the stitch increases so that the finished piece looked like a circle and not like a hexagon), I realized that I would need to so something about the edges so that the edge did not curl so much:

I tried several things.
I continued working a final round with a slip stitch.
That did not work.
I frogged that and tried working the final round in something I call a half double slip stitch (hdss).
That also did not work.
I frogged that, and then (because of something I had noticed when working the half double slip stitch while looking at the front of the cookie, I tried working the half-double slip stitch in the opposite direction with the back of the cookie facing me.
To my delight (and relief), it worked, and the “cookie” was much better behaved:

Eventually, I finished edging the second cookie as well. By then, both pieces needed to be soaked in water, dried by rolling them out between towels, and then placed under a ceiling fan to dry.
And while the cookies dried, I added an 11th row to the first shawl:

wove in a few of the ends of the tenth row:

and made a quick video that demonstrates the magic (and ease) of the half double slip stitch.
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Thanks for your explanation of the half double slip stitch. I had never heard of it before. (And I ask myself why I never thought of it.)
But I have one question. The half double slip stitch in the opposite direction might be more decorative but did you try a row of slip stitches or single crochet in the opposite to avoid the curling?