If you want to knit a hat, you're going to need yarn, the thick string made of twisted threads that's used to make sweaters, scarves, socks and other knitted things.
Yarn is made from fibers, and it comes in many colors. When you knit, you weave yarn with needles and end up with a pot holder or a pair of mittens. Another kind of yarn is a long story that's told out loud. You might, in fact, tell your friend a yarn while you crochet an afghan with yarn. The Old English root word, gearn, means "spun fibers," but it comes from the Proto-Indo-European ghere, or "intestine."