Skewers are straight and pretty tough. But put just the right twist on them, and you can make something surprisingly curvy!

## You will need

- Sponge cloth – ours was about 0.5 centimetres (cm) thick and about 20 cm wide
- 16 bamboo skewers
- Marker
- Mug or other circular object to trace
- Ruler (optional)

## What to do

- Trace 2 circles onto the sponge cloth.
- Leave a gap around each circle and cut them out.
- Draw a vertical line through the centre of one circle, cutting it in half.
- Draw a horizontal line through the circle, dividing it into 4 equal quarters.
- Now divide each quarter in half to make 8 equal pizza slice segments.
- Finally, divide each of these slices in half once again. You will end up with 16 equal segments.
- Starting at the top of the circle, number each point where a straight line meets the circle.
- Repeat steps 3–7 on the other circle.
- Line up the point of a skewer with point 1 on one of the circles.
- Roll the skewer between your fingers to make it spin a bit like a drill and gently poke it through the sponge cloth. Don’t force it! Keep going until most of the skewer has poked through.
- Take your second circle, and do the same thing, gently drilling through the point marked 1.
- Adjust the 2 sponge circles so they are about 3 cm from each end of the skewer.
- Take a new skewer and gently drill it through point 2 on the first circle, then the second circle. The 2 skewers should be parallel.
- Repeat this process for each number around your circles. You will end up with a cylinder shape made of skewers.
- Hold one sponge in each hand and twist them in different directions. What shape do the skewers make?

## What’s happening?

You may be surprised to make a curvy shape out of straight skewers. But such shapes are not as rare as you might think.

A 3D shape that can be made out of straight lines is called a ruled surface. There are many different types of ruled surface, including cylinders and helixes – like the underside of a spiral staircase.

The shape in this activity is called a hyperboloid. It is a doubly ruled surface, because there are 2 ways of making a hyperboloid out of straight lines. You could twist the lines clockwise, or you could twist them anticlockwise. Sydney Tower, or Centrepoint, is supported by cables that make a hyperboloid shape. These cables run both clockwise and anticlockwise – showing the doubly ruled surface.
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