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Get a plain T-shirt without any side seams. Cut the top of the shirt off below the armpits. Cut the hem off of the shirt. Fold the shirt in half, leaving a gap between the left and right edges. Cut 1-inch (2.5-cm) slits into the side of the shirt, stopping at the gap. Unfold the shirt and spread it so that gap in the right side edge widens. Cut diagonal lines across the 2-inch (5.1-cm) gap to separate the strips. Pull on the T-shirt strip to make the ends curl in. Roll the yarn into a ball. Use the yarn to knit or crochet.
If the shirt has side seams, they will come apart once you go to cut the T-shirt into yarn. Also, do not use a T-shirt that has an image printed onto it, like a band logo or a comic book character, as this will prevent the shirt from stretching. Spread your T-shirt on a flat surface. Place a ruler across the shirt, just under the armpits. Cut across the shirt with a rotary cutter, using the ruler as a guide. Discard the top part of the shirt, along with the sleeves. If you don't have a rotary cutter, use the ruler and a marker to draw a horizontal line instead, then cut across the line with fabric scissors. While not absolutely necessary, this will help make your yarn more consistent in thickness and texture. Simply cut straight across the bottom hem just above the stitching. You can do this with a ruler and rotary cutter, or with fabric scissors. Discard the hem when you are done. You don't need it. Take the left side of the shirt and pull it towards the right. Stop when there is a 1-inch (2.5-cm)  gap between the left side edge and the right side edge. This will reduce the amount of cutting that you have to do. Begin cutting on the left side of the shirt, and finish cutting 1 inch (2.5 cm) from the right side. If you were to unfold your shirt, you would see 1-inch (2.5-cm) wide strips running across it, and stopping 1 inch (2.5 cm) from the right side edge. Unfold the shirt first. Next, open it up and spread it out so that the 1-inch (2.5-cm) gap widens into a 2-inch (5.1-cm) gap. You may have to move the loops formed by the cut left side of the shirt out of the way. Begin cutting at the bottom-left corner of the gap. Cut across the gap at an upward angle towards the next horizontal strip. Repeat this process, cutting from the bottom-left to the upper-right until you reach the top of the shirt. Envision the gaps as 1-inch (2.5-cm) tall rectangles. You are cutting from the bottom-left corner to the upper-right corner of each rectangle. Hold the T-shirt strip loosely in 1 hand, and use your other hand to gently pull it through your hand. This will cause the edges of the T-shirt to curl inwards and form a tube-like shape. You may need to do this a few times. Wrap the end of the yarn a few times around your index and middle fingers. Slide the wrapped yarn off your fingers, then wrap the yarn across the loop a few more times to form a ball. Continue wrapping the yarn around the ball, changing direction often, until you run out. Because of how thick and bulky this yarn is, it probably won't make very good socks, hats, or scarves. It is great for making items that will see a lot of wear-and-tear, such as dishcloths, pot holders, and rugs, however.