The term 'Knot in Yarn' can be summarized as a four-letter word, 'BURL'. A BURL is a knot or lump of tangled fibers that form in yarn or fabric. It is usually caused by a disruption in the spinning process, such as a twist or snag in the yarn. Burls can also be caused by the presence of foreign objects, such as dirt or debris, in the yarn. Burls can be difficult to remove and can affect the quality of the finished product.