All yarn fibers can be divided into four main categories - animal fibers (such as wool, silk, and alpaca), plant fibers (such as cotton and linen), biosynthetic fibers (such as rayon and bamboo), and synthetic fibers (such as acrylic and Nylon). Acrylic yarn falls into the synthetic fiber category because it is a petroleum-based product that has been designed and manufactured to be a soft, lightweight, and durable yarn. It's not a natural fiber - it's man-made.

　　The process of making acrylic yarn begins by melting and mixing the "ingredients" and extruding them through a spinneret to form bundled filaments. These strands are then drawn and cut to mimic the staple lengths of other fibers before being combed and spun into yarn. In the end, this blend of petroleum-based man-made materials ends up looking and feeling like real natural fibers.

　　Every acrylic yarn is different. Some may feel soft when braided, others may feel scratchy. Some will feel slippery, others won't. It just depends on how the filament is cut and spun and what the acrylic is mixed with. Therefore, you need to choose your knitting needles carefully.

　　For smooth fibers, knitting with plastic, wooden, or bamboo needles may be a better choice - something with some "grip" so that your stitches don't slip and slip off the needles. For less smooth fibers, you can use any type of needle material you like.