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String is floppy, so it’s usually a terrible thing to build a tower from. But this tower uses tension and string to hold itself strong!

You will need

• 4 pipe cleaners (30 cm long)
• String
• Scissors
• Ruler

What to do

1. Bend a pipe cleaner into a circle, and wrap the ends around each other to make it nice and strong. Our circle ended up about 9 centimetres across.
2. Make a second circle like the first.
3. Take a third pipe cleaner and find the midpoint. Put three zig-zag bends near the middle of the pipe cleaner, about five millimetres apart from each other. It should look a bit like a volcano – an upside-down V with a crater at the top.
4. Make a copy of the volcano shape with the last pipe cleaner.
5. Use one of the circles to act as a base for one of the volcano shapes. Put the circle inside the volcano shape, adjusting the volcano so it’s about 9 centimetres tall. Then twist the volcano pipe cleaner onto opposite sides of the circle to hold it in place.
6. Cut three pieces of string, each about 20 centimetres long.
7. Tie each piece of string to a different place, equally spaced around the circle at the base of the volcano.
8. Loosely tie the three strings to the second circle. Adjust the strings so they’re each about 12 centimetres long. Then do fine adjustments so they are all same length, and the volcano base sits level if you hang it below the top circle.
9. Now, attach the remaining volcano pipe cleaner to the top circle, only upside down. You want the point of this volcano to go under the volcano on the base. For the best results, you want the bottom volcano and top volcano to pass through each other, linking like a chain.
10. Almost there! Cut another piece of string about 20 centimetres long. Tie it into a loop between the points of the two volcanoes.
11. Slowly tighten the loop until the structure stands up by itself. This may require some patience and adjustment of the string and pipe cleaners.
12. If you look at the top circle and upside volcano, they don’t touch the table, and they’re only held up with string!
13. When you’re finished, trim any excess string to make the tower nice and neat.

What’s happening?

It’s a fact of life that some things are stronger than others. Metal spoons are stronger than disposable spoons, and cardboard is stronger than tissue paper. But some comparisons are harder to make. Riddle me this: is a pile of rocks stronger than rope?

There is more than one kind of strength. A pile of rocks is strong when you squish it. You might be able to stand on a well-built pile without disturbing it at all. A rope is strong when you pull on it – tie it securely to a tree and you’ll be able to swing.

But reverse the roles and you run into problems. A rope is so floppy it can’t even hold itself up. And there’s absolutely no way to pull on a pile of rocks without breaking it.

In this activity, there are two shapes made from pipe cleaners. They don’t touch each other, and the top shape doesn’t touch the table. Instead, the top shape is supported by string!

String is only strong when it is in tension – when it is being pulled on. In this tower, the pieces of string around the circles are pulling the top shape down, and the string in the middle is pulling it up. The forces pulling in opposite directions are balanced, and this makes the whole structure strong.

Real-life engineering

Balancing opposite forces is often used in engineering. Many buildings use prestressed concrete, which is concrete with tightened steel rods or cables running through it. The steel squeezes (or compresses) the concrete, and the concrete stretches (or tensions) the steel, and the two forces make the whole structure very strong.

Also in this newsletter

Afraid of the dark brainteaser
Protecting paintings with flow chemistry
Which is bigger? A quick quiz

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