"What is Felt? Very simply, felt is matted wool. Wool becomes felt when it is subjected to moisture, heat, and pressure. (In fact, if domestic sheep were not shorn, over time their wool would felt or "cot".) Hot soapy water makes the wool slippery, and causes tiny scales on the fiber to "open up". The scales prevent the fibers from backing up again after they slide across each other; with agitation, the fibers get hopelessly tangled together. When cooled and dried, the scales close and lock the wool into the tough, durable material we call felt."
Here's a close up of the felted dryer balls I made last year. It's a lot of fun to make them. Just roll 100% wool yarn in a small, tight ball. Stick in a pair of panty hose. Tie a knot before adding another ball. I used an old pair of knee highs & put about 4 balls in each one. Then run them through the washer & dryer with several loads of your clothes. Hot water & a hot dryer temp are good, too. After doing that a few times the yarn will start to look like felt & all stick together. Then you can take them out of the panty hose & use them in your dryer to fluff up your clothes.
I've also run a few old wool sweaters through the washer & dryer. Just getting ready for some projects. But I haven't made any yet. I've bought some fun old wool sweaters at the thrift store for $1-3 that I wouldn't mind cutting apart.
I did buy this book with lots of ideas of felted things to make. I love Betz White's books & her blog. Lots of really cute ideas!