Cardboard or posterboard, scissors, yarn.
Make a pom-pom to play with or tie onto a hair rubberband. Cut out 2 "donuts" using cardboard or posterboard - you can also use the outside of a cereal box. Measure out about 15 yards of yarn (depending on thickness) and fold it in two. Next lay the "donuts" on top of each other and wrap the folded-over yarn through the holes, around and around until you run out of yarn and the "donut" is covered with yarn. Take scissors and cut the yarn in between the 2 donuts. Carefully pull out the "donuts" and tie around the center with some extra yarn several times so it's very tight - this yarn will be holding the whole thing together. Now fluff up your ball of yarn into a pom-pom.