Explain to the students that the ball of yarn represents the energy in the environment. Ask the student who represents the sun to hold the end of the yarn tightly and toss the ball to someone who can use that energy. When that student catches the yarn, they should pass the yarn to someone else who could use the energy. (Example: Sun->green plant->rabbit->fox) The ball of yarn should then be returned to the sun and repeated until every student is part of the food web.