Cut a piece of yarn that's about 18 to 24 inches long. The shorter the yarn, the easier it will be to work with.
Soak the piece of yarn in water for a few minutes to help the paint absorb in to the string.
Pour the watercolor paint into a small bowl. It should have at least 1/4 inch of paint in the bottom. To make lighter colors, add a bit of water to the paint to dilute it.
Place the yarn into the bowl of paint. Use a fork or a popsicle stick to completely saturate the yarn with the paint. Leave about 4 inches of clean yarn outside the bowl so you can pick it up later.
Fold a piece of printer paper in half like a book. Open it, then pull the string out of the paint and lay it out on one side of the paper so that the clean end comes out at the bottom.
Fold the paper so it's closed with the string inside. Place a tissue box on top of the paper to hold everything in place.
Gently press down the top of the tissue box onto the paper. Pull the string straight out from between the paper, keeping it straight and in the same position as much as possible.
Once the entire string has been removed, remove the tissue box and unfold your paper. You'll have mirror images of your design on each side!
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Notes
To make your painting different colours, dip one piece of string into three different bowls of paint. Or you can wait for the paint to dry, then add a second string painting on top of the first one.