I've been learning to crochet. I'm trying to do more hobbies with my hands, but it's also pretty interesting from a mathematical perspective. The fundamental primitive (the chain stitch) is like a series of slip knots, and each stitch is reversible. So the piece is actually a series of reversible transformations. The yarn is sewn in at the end to secure it.

This has some interesting implications. If you make a mistake, you can always backtrack and try again. If you have a crocheted piece, at least in principle you could find the lose end, free it, and work back stitch by stitch to reverse engineer it. (In practice people don't seem to do a stitch-for-stitch reverse engineering just like you probably wouldn't bother reimplementing something line by line without a compelling reason, you figure out what's going on in the challenging places just by look and feel and improvise from there.)

I'm oversimplifying somewhat and there are some forms of crochet that include irreversible stitches, yarn can be felted together (entangled, like a cotton ball) to create irreversible bonds between adjacent strands, and often several panels/pieces are joined together irreversibly to create a larger piece.

I guess I should be clear that by "irreversible" I mean a transformation like the following: "to cut the yarn with scissors, to untie a knot that was strongly bound, or to felt together." So a slip knot is "reversible" in the sense that if you tug on it, it easily comes undone, whereas an overhand knot would just get tighter. You can think of felting as being equivalent to tying a lot of overhand knots between adjacent strands, they become permanently attached and could only be torn from each other.