A simple way to draw circles and parts of circles when making designs is to use a pin and a loop of string. Use a pencil to stretch the string straight and trace out the circle. Tying knots in different places along the string will give different size circles.
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But have you ever tried two pins? Again use a pencil to stretch the string out straight and keep it stretched tight as you carefully move the pencil around. What shape do you think you'll get? Try it!
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Try changing the distance between the pins. Try changing the length of the string. How does this affect the shape that is drawn?
(Tip: Don't stick pins into a table - adults tend to get upset. Try drawing pins pushed into a thick layer of newspaper. A sheet of thick cardboard from a box works well too, but you might like to use a felt tip pen instead of a pencil.)