Something as simple as a string can be used. If you hold down one end of the string onto a piece of paper and tie the other end onto a pencil, rotating your pencil around the center while keeping the string taught will create a circle. 
Creating a circle without any tool but a pencil is possible but might be difficult. One possible way is to use your wrist bone as a pivot point on paper and spin the paper around underneath it while keeping the pencil still.
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