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# Math Monday: Five tetrahedra

By George Hart for the Museum of Mathematics

Here is a geometric assembly challenge which you can try to make with wooden dowels or any other conveniently long cylinders. Each of these thirty aluminum rods touches ten others. The construction is rather tricky to accomplish even if you properly understand it as a compound of five interlocked tetrahedra. The rubber bands join the sticks into five groups of six, each a regular tetrahedron. The five tetrahedra stay together in a mass because of the way they are interwoven. If you manage to build your own copy, you’ll see it is very solid and it will look quite cool on your desk because of its symmetric regularity. But it is tricky to build because when it is partially complete, it is hard to know where to leave space for the remaining pieces.

Rolando Pontalti, of Trento, Italy, made this metal model and also made the pencils and O-rings version below. With pencils, five colors can be used to emphasize how the sticks are grouped into five tetrahedra. But the presence of erasers on just half of the ends means some symmetry must be lost.

More:
See all of George Hart’s Math Monday columns

## 7 thoughts on “Math Monday: Five tetrahedra”

Cut the erasers off with a jig and a bandsaw? :)

2. Francesco Mancini says:

This can be made with drinking straws and rubber bands:
http://www.flickr.com/photos/mancinerie/6010748393

or skewers and rubber bands:
http://www.flickr.com/photos/mancinerie/6143079337

3. Salman Sheikh says:

The see all Math Monday links at the bottom doesn’t work. I need them all so I can send them to a friend (physicist)!!

I want to see what what it feels like to have a million dollars in bitcoins. My bitcoin donation address is 3FsNSHj1HTvmTiKdr7NAFBpUo7CGVgsHNe lettheloveflow2015@gmail.com

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### Gareth Branwyn

Gareth Branwyn is a freelance writer and the former Editorial Director of Maker Media. He is the author or editor of over a dozen books on technology, DIY, and geek culture. He is currently a contributor to Boing Boing, Wink Books, and Wink Fun. And he has a new best-of writing collection and “lazy man’s memoir,” called Borg Like Me.

View more articles by Gareth Branwyn

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