# Blog

## Decorative octahedron

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In this crafty maths activity, you’ll get to make a decoration in the shape of an octadehron.

#### You will need

• Drinking straws
• Scissors
• String or yarn
• Tapestry needle (optional)

#### What to do

1. Cut 8 short pieces of straw, each 4 cm long.
2. Cut 4 long pieces of straw, each 8 cm long.
3. Cut a piece of string about 2 m long, and thread it through your needle (if you have one).
4. Thread four short pieces of straw onto the string. Pull them into a tight loop and use a knot to hold the straws in a square.

5. Thread two more short pieces onto the string, and tie the string to the opposite corner of the square. It will be too long to lie flat.
6. Feed the string through one of the straws in the square, so you end up at a different corner. Thread a new short straw onto the string, and then tie this between the most recently added short straws.
7. Thread on the final short straw and then tie this to the far corner – the only corner that doesn’t already have three straws going to it. Once you have tightened everything, you should have a pyramid shape.
8. Turn the pyramid upside-down.
9. Thread on two longer straws and then tie the string to the opposite corner of the original square.
10. Feed the string through one of the straws in the original square.

11. Thread on another long straw and tie it in between the two other long straws.
12. Thread on the final long straw and tie this to the remaining corner.
13. Thread the string through one of the short straws to the top of the original pyramid. Use this string to hang your decoration!

#### What’s happening?

The shape you’ve made is an octahedron. An octahedron is a three dimensional shape with eight faces. The word octahedron is made of two parts – octa- meaning eight, and -hedron describing a solid, three dimensional object.

There are many different types of octahedron. The most famous is the regular octahedron, which is like the one you made, only the edges are all the same length. Regular octahedrons are found in some crystals, and can be used to make eight-sided dice!

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